diff --git a/projects/packages/jetpack-mu-wpcom/changelog/remove-start-writing-checklist b/projects/packages/jetpack-mu-wpcom/changelog/remove-start-writing-checklist
new file mode 100644
index 000000000000..d1e41d04072f
--- /dev/null
+++ b/projects/packages/jetpack-mu-wpcom/changelog/remove-start-writing-checklist
@@ -0,0 +1,4 @@
+Significance: minor
+Type: removed
+
+Remove the deprecated start-writing launchpad checklist and its remaining task handling.
diff --git a/projects/packages/jetpack-mu-wpcom/src/features/launchpad/launchpad-task-definitions.php b/projects/packages/jetpack-mu-wpcom/src/features/launchpad/launchpad-task-definitions.php
index 59037d14c277..ab7dd04bffa8 100644
--- a/projects/packages/jetpack-mu-wpcom/src/features/launchpad/launchpad-task-definitions.php
+++ b/projects/packages/jetpack-mu-wpcom/src/features/launchpad/launchpad-task-definitions.php
@@ -1833,9 +1833,9 @@ function wpcom_track_site_launch_task() {
 	wpcom_launchpad_mark_launchpad_task_complete_if_active( 'videopress_launched' );
 	wpcom_launchpad_mark_launchpad_task_complete_if_active( 'blog_launched' );
 
-	// Remove site intent for blog onboarding flows and disable launchpad.
+	// Remove site intent for the blog onboarding flow and disable launchpad.
 	$site_intent = get_option( 'site_intent' );
-	if ( in_array( $site_intent, array( 'start-writing', 'design-first' ), true ) ) {
+	if ( 'design-first' === $site_intent ) {
 		update_option( 'site_intent', '' );
 		update_option( 'launchpad_screen', 'off' );
 	}
@@ -1996,15 +1996,6 @@ function wpcom_launchpad_is_blog_launched_task_disabled() {
 		}
 		return true;
 	}
-	if ( 'start-writing' === get_option( 'site_intent' ) ) {
-		if ( wpcom_is_checklist_task_complete( 'plan_completed' )
-			&& wpcom_is_checklist_task_complete( 'domain_upsell' )
-			&& wpcom_is_checklist_task_complete( 'setup_blog' )
-			&& wpcom_is_checklist_task_complete( 'first_post_published' ) ) {
-			return false;
-		}
-		return true;
-	}
 	return false;
 }
 
@@ -2819,7 +2810,7 @@ function wpcom_trigger_email_campaign() {
 		return;
 	}
 
-	if ( ! in_array( $site_intent, array( 'start-writing', 'design-first' ), true ) ) {
+	if ( 'design-first' !== $site_intent ) {
 		return;
 	}
 
diff --git a/projects/packages/jetpack-mu-wpcom/src/features/launchpad/launchpad.php b/projects/packages/jetpack-mu-wpcom/src/features/launchpad/launchpad.php
index ebab614cfade..129e7810427e 100644
--- a/projects/packages/jetpack-mu-wpcom/src/features/launchpad/launchpad.php
+++ b/projects/packages/jetpack-mu-wpcom/src/features/launchpad/launchpad.php
@@ -151,19 +151,6 @@ function wpcom_launchpad_get_task_list_definitions() {
 			),
 			'is_enabled_callback' => 'wpcom_launchpad_get_fullscreen_enabled',
 		),
-		'start-writing'           => array(
-			'get_title'           => function () {
-				return __( 'Next steps for your site', 'jetpack-mu-wpcom' );
-			},
-			'task_ids'            => array(
-				'first_post_published',
-				'setup_blog',
-				'domain_upsell',
-				'plan_completed',
-				'blog_launched',
-			),
-			'is_enabled_callback' => 'wpcom_launchpad_get_fullscreen_enabled',
-		),
 		'design-first'            => array(
 			'get_title'           => function () {
 				return __( 'Next steps for your site', 'jetpack-mu-wpcom' );
@@ -440,6 +427,20 @@ function wpcom_register_default_launchpad_checklists() {
 }
 add_action( 'init', 'wpcom_register_default_launchpad_checklists', 11 );
 
+/**
+ * The site intents of retired onboarding flows.
+ *
+ * These no longer have a checklist, but sites created under them still carry
+ * the value, and clients still pass it to the launchpad endpoint as a checklist
+ * slug. The endpoint keeps accepting these so such a request degrades to an
+ * empty checklist rather than a 400. Empty this list once no site carries one.
+ *
+ * @return string[] Retired site intents.
+ */
+function wpcom_launchpad_get_retired_site_intents() {
+	return array( 'start-writing' );
+}
+
 /**
  * Clears the site intent for a retired onboarding flow.
  *
@@ -452,9 +453,7 @@ function wpcom_register_default_launchpad_checklists() {
  * once no site carries one of these values.
  */
 function wpcom_launchpad_clear_retired_site_intents() {
-	$retired_site_intents = array( 'start-writing' );
-
-	if ( ! in_array( get_option( 'site_intent' ), $retired_site_intents, true ) ) {
+	if ( ! in_array( get_option( 'site_intent' ), wpcom_launchpad_get_retired_site_intents(), true ) ) {
 		return;
 	}
 
@@ -1007,24 +1006,34 @@ function wpcom_launchpad_navigator_remove_checklist( $checklist_slug ) {
 		);
 	}
 
-	$current_active_checklist = wpcom_launchpad_get_active_checklist();
-
 	$checklists = $wpcom_launchpad_config['navigator_checklists'];
 	// Find if $checklist_slug is in the checklists array. If it is, remove it.
 	$key = array_search( $checklist_slug, $checklists, true );
 	if ( $key === false ) {
 		return array(
 			'updated'              => false,
-			'new_active_checklist' => $current_active_checklist,
+			'new_active_checklist' => wpcom_launchpad_get_active_checklist(),
 		);
 	}
 
 	unset( $checklists[ $key ] );
 
-	$new_active_checklist = $current_active_checklist;
-	if ( $current_active_checklist === $checklist_slug ) {
-		// get last item on $checklists array, if there is one; otherwise set to null
-		$new_active_checklist = end( $checklists ) ? end( $checklists ) : null;
+	// Compare against the raw stored slug, not the getter: a retired active checklist
+	// reads as null there, which would otherwise hide that it was the one removed.
+	$stored_active_slug = $wpcom_launchpad_config['active_checklist_slug'] ?? null;
+
+	$new_active_checklist = wpcom_launchpad_get_active_checklist();
+	if ( $stored_active_slug === $checklist_slug ) {
+		// The active checklist was removed. Promote the most recently added one that
+		// is still registered, skipping any other retired slugs, or clear it.
+		$registered_checklists = wpcom_launchpad_checklists()->get_all_task_lists();
+		$new_active_checklist  = null;
+		foreach ( array_reverse( $checklists ) as $remaining_slug ) {
+			if ( is_string( $remaining_slug ) && array_key_exists( $remaining_slug, $registered_checklists ) ) {
+				$new_active_checklist = $remaining_slug;
+				break;
+			}
+		}
 		wpcom_launchpad_set_current_active_checklist( $new_active_checklist );
 	}
 
@@ -1073,13 +1082,28 @@ function wpcom_launchpad_get_active_checklist() {
 		return null;
 	}
 
-	return $wpcom_launchpad_config['active_checklist_slug'];
+	$active_checklist_slug = $wpcom_launchpad_config['active_checklist_slug'];
+
+	// This repairs persisted state, so it cannot assume a well-formed value: legacy
+	// or custom code may have stored a non-string, which would fatal below.
+	if ( ! is_string( $active_checklist_slug ) ) {
+		return null;
+	}
+
+	// A retired checklist can linger here from before it was unregistered. Treat it
+	// as unset, matching how the available list drops unregistered slugs and how the
+	// setter refuses them, so the Navigator never reports a checklist that is gone.
+	if ( ! array_key_exists( $active_checklist_slug, wpcom_launchpad_checklists()->get_all_task_lists() ) ) {
+		return null;
+	}
+
+	return $active_checklist_slug;
 }
 
 /**
  * Helper function to set the current active checklist in the navigator context.
  *
- * @param string $checklist_slug The slug of the launchpad task list to mark as active.
+ * @param string|null $checklist_slug The slug of the launchpad task list to mark as active, or null to clear it.
  * @return bool Whether the option update succeeded.
  */
 function wpcom_launchpad_set_current_active_checklist( $checklist_slug ) {
diff --git a/projects/packages/jetpack-mu-wpcom/src/features/wpcom-endpoints/class-wpcom-rest-api-v2-endpoint-launchpad-navigator.php b/projects/packages/jetpack-mu-wpcom/src/features/wpcom-endpoints/class-wpcom-rest-api-v2-endpoint-launchpad-navigator.php
index b39ce8b4b0ec..8025ea2b8738 100644
--- a/projects/packages/jetpack-mu-wpcom/src/features/wpcom-endpoints/class-wpcom-rest-api-v2-endpoint-launchpad-navigator.php
+++ b/projects/packages/jetpack-mu-wpcom/src/features/wpcom-endpoints/class-wpcom-rest-api-v2-endpoint-launchpad-navigator.php
@@ -50,7 +50,7 @@ public function register_routes() {
 						'remove_checklist_slug' => array(
 							'description' => 'The slug of the checklist to remove from the active list.',
 							'type'        => 'string',
-							'enum'        => $this->get_checklist_slug_enums(),
+							'enum'        => $this->get_readable_checklist_slug_enums(),
 						),
 					),
 				),
@@ -61,7 +61,7 @@ public function register_routes() {
 	/**
 	 * Validates that the argument sent to the active_checklist_slug parameter is a valid checklist slug or empty.
 	 *
-	 * @param string $value The value of the active_checklist_slug parameter.
+	 * @param string|null $value The value of the active_checklist_slug parameter.
 	 * @return bool
 	 */
 	public function validate_checklist_slug_param( $value ) {
@@ -83,6 +83,21 @@ public function get_checklist_slug_enums() {
 		return array_keys( $checklists );
 	}
 
+	/**
+	 * Returns the checklist slugs a read or removal request may reference.
+	 *
+	 * Retired flow slugs are included so a stale client can still read or remove
+	 * one. Setting a retired slug as active still uses the registered slugs from
+	 * get_checklist_slug_enums(), so it is rejected at the boundary.
+	 *
+	 * @return array Array of checklist slugs.
+	 */
+	public function get_readable_checklist_slug_enums() {
+		return array_values(
+			array_unique( array_merge( $this->get_checklist_slug_enums(), wpcom_launchpad_get_retired_site_intents() ) )
+		);
+	}
+
 	/**
 	 * Updates Launchpad navigator-related options and returns the result
 	 *
diff --git a/projects/packages/jetpack-mu-wpcom/src/features/wpcom-endpoints/class-wpcom-rest-api-v2-endpoint-launchpad.php b/projects/packages/jetpack-mu-wpcom/src/features/wpcom-endpoints/class-wpcom-rest-api-v2-endpoint-launchpad.php
index 6ad250fb1849..bd6f171a9e8f 100644
--- a/projects/packages/jetpack-mu-wpcom/src/features/wpcom-endpoints/class-wpcom-rest-api-v2-endpoint-launchpad.php
+++ b/projects/packages/jetpack-mu-wpcom/src/features/wpcom-endpoints/class-wpcom-rest-api-v2-endpoint-launchpad.php
@@ -43,7 +43,7 @@ public function register_routes() {
 						'checklist_slug'             => array(
 							'description' => 'Checklist slug',
 							'type'        => 'string',
-							'enum'        => $this->get_checklist_slug_enums(),
+							'enum'        => $this->get_readable_checklist_slug_enums(),
 						),
 						'launchpad_context'          => array(
 							'description' => 'Screen where Launchpand instance is loaded.',
@@ -122,6 +122,21 @@ public function get_checklist_slug_enums() {
 		return array_keys( $checklists );
 	}
 
+	/**
+	 * Returns the checklist slugs a read request may reference.
+	 *
+	 * Retired flow slugs are included so a stale client request degrades to an
+	 * empty checklist rather than a 400. Read-only: mutations use the registered
+	 * slugs from get_checklist_slug_enums() so they cannot act on a retired slug.
+	 *
+	 * @return array Array of checklist slugs.
+	 */
+	public function get_readable_checklist_slug_enums() {
+		return array_values(
+			array_unique( array_merge( $this->get_checklist_slug_enums(), wpcom_launchpad_get_retired_site_intents() ) )
+		);
+	}
+
 	/**
 	 * Returns all registered checklist statuses.
 	 *
@@ -190,7 +205,7 @@ public function get_data( $request ) {
 			'launchpad_screen'   => get_option( 'launchpad_screen' ),
 			'checklist_statuses' => get_option( 'launchpad_checklist_tasks_statuses', array() ),
 			'checklist'          => wpcom_get_launchpad_checklist_by_checklist_slug( $checklist_slug, $launchpad_context ),
-			'is_enabled'         => wpcom_get_launchpad_task_list_is_enabled( $checklist_slug ),
+			'is_enabled'         => (bool) wpcom_get_launchpad_task_list_is_enabled( $checklist_slug ),
 			'is_dismissed'       => wpcom_launchpad_is_task_list_dismissed( $checklist_slug ),
 			'is_dismissible'     => wpcom_launchpad_is_task_list_dismissible( $checklist_slug ),
 			'title'              => wpcom_get_launchpad_checklist_title_by_checklist_slug( $checklist_slug ),
diff --git a/projects/packages/jetpack-mu-wpcom/tests/php/features/launchpad/Launchpad_Retired_Site_Intents_Test.php b/projects/packages/jetpack-mu-wpcom/tests/php/features/launchpad/Launchpad_Retired_Site_Intents_Test.php
index 5285f1e4c0c8..792fa9146ba8 100644
--- a/projects/packages/jetpack-mu-wpcom/tests/php/features/launchpad/Launchpad_Retired_Site_Intents_Test.php
+++ b/projects/packages/jetpack-mu-wpcom/tests/php/features/launchpad/Launchpad_Retired_Site_Intents_Test.php
@@ -5,6 +5,9 @@
  * @package automattic/jetpack-mu-wpcom
  */
 
+//phpcs:ignore WordPressVIPMinimum.Files.IncludingFile.NotAbsolutePath
+require_once \Automattic\Jetpack\Jetpack_Mu_Wpcom::PKG_DIR . 'src/features/wpcom-endpoints/class-wpcom-rest-api-v2-endpoint-launchpad-navigator.php';
+
 use PHPUnit\Framework\Attributes\DataProvider;
 
 /**
@@ -208,4 +211,91 @@ public function test_cleanup_ignores_the_intent_seen_before_the_blog_switch() {
 		$this->assertSame( 'design-first', get_option( 'site_intent' ), 'the switched-to blog must survive' );
 		$this->assertSame( 'full', get_option( 'launchpad_screen' ), 'the switched-to blog must survive' );
 	}
+
+	/**
+	 * A Navigator config left pointing at a retired checklist reports no active
+	 * checklist, rather than one that no longer exists. This can outlive the
+	 * intent cleanup, so it must not depend on `site_intent` still matching.
+	 */
+	public function test_retired_active_checklist_slug_reads_as_none() {
+		wpcom_register_default_launchpad_checklists();
+		// Cleared already, to prove the repair does not depend on the intent still matching.
+		update_option( 'site_intent', '' );
+		update_option(
+			'wpcom_launchpad_config',
+			array(
+				'active_checklist_slug' => 'start-writing',
+				'navigator_checklists'  => array( 'start-writing', 'design-first' ),
+			)
+		);
+
+		$this->assertNull( wpcom_launchpad_get_active_checklist(), 'retired active slug must read as none' );
+	}
+
+	/**
+	 * A live active checklist is still reported.
+	 */
+	public function test_live_active_checklist_slug_is_reported() {
+		wpcom_register_default_launchpad_checklists();
+		update_option(
+			'wpcom_launchpad_config',
+			array( 'active_checklist_slug' => 'design-first' )
+		);
+
+		$this->assertSame( 'design-first', wpcom_launchpad_get_active_checklist() );
+	}
+
+	/**
+	 * Removing a retired active checklist promotes the next registered one, rather
+	 * than leaving the retired slug stored with no active checklist reported.
+	 */
+	public function test_removing_retired_active_checklist_promotes_next_registered() {
+		wpcom_register_default_launchpad_checklists();
+		update_option(
+			'wpcom_launchpad_config',
+			array(
+				'active_checklist_slug' => 'start-writing',
+				'navigator_checklists'  => array( 'start-writing', 'design-first' ),
+			)
+		);
+
+		$result = wpcom_launchpad_navigator_remove_checklist( 'start-writing' );
+
+		$this->assertSame( 'design-first', $result['new_active_checklist'], 'the next registered checklist must be promoted' );
+		$this->assertSame( 'design-first', wpcom_launchpad_get_active_checklist(), 'the promotion must be persisted' );
+	}
+
+	/**
+	 * A non-string active checklist slug from malformed legacy state reads as none
+	 * rather than fataling on the registered-checklist lookup.
+	 */
+	public function test_malformed_active_checklist_slug_reads_as_none() {
+		wpcom_register_default_launchpad_checklists();
+		update_option(
+			'wpcom_launchpad_config',
+			array( 'active_checklist_slug' => array( 'unexpected' ) )
+		);
+
+		$this->assertNull( wpcom_launchpad_get_active_checklist() );
+	}
+
+	/**
+	 * Retired slugs are accepted for reads and removal, but not for setting the
+	 * active checklist, so a retired flow cannot be re-selected.
+	 */
+	public function test_navigator_enums_separate_reads_from_mutations() {
+		wpcom_register_default_launchpad_checklists();
+		$navigator = new WPCOM_REST_API_V2_Endpoint_Launchpad_Navigator();
+
+		$registered = $navigator->get_checklist_slug_enums();
+		$this->assertContains( 'design-first', $registered );
+		$this->assertNotContains( 'start-writing', $registered, 'mutations must not accept a retired slug' );
+
+		$readable = $navigator->get_readable_checklist_slug_enums();
+		$this->assertContains( 'start-writing', $readable, 'reads and removal must accept a retired slug' );
+
+		$this->assertFalse( $navigator->validate_checklist_slug_param( 'start-writing' ), 'setting a retired slug active must be rejected' );
+		$this->assertTrue( $navigator->validate_checklist_slug_param( 'design-first' ) );
+		$this->assertTrue( $navigator->validate_checklist_slug_param( null ), 'clearing the active checklist must stay allowed' );
+	}
 }
diff --git a/projects/packages/jetpack-mu-wpcom/tests/php/features/wpcom-endpoints/WPCOM_REST_API_V2_Endpoint_Launchpad_Test.php b/projects/packages/jetpack-mu-wpcom/tests/php/features/wpcom-endpoints/WPCOM_REST_API_V2_Endpoint_Launchpad_Test.php
index 9705aa858578..ed42feb67a73 100644
--- a/projects/packages/jetpack-mu-wpcom/tests/php/features/wpcom-endpoints/WPCOM_REST_API_V2_Endpoint_Launchpad_Test.php
+++ b/projects/packages/jetpack-mu-wpcom/tests/php/features/wpcom-endpoints/WPCOM_REST_API_V2_Endpoint_Launchpad_Test.php
@@ -72,6 +72,47 @@ public function test_get_data() {
 		$this->assertFalse( $result->get_data()['is_dismissed'] );
 	}
 
+	/**
+	 * A retired flow slug has no checklist, but clients still pass it explicitly
+	 * from stale site options. It must degrade to a complete, well-typed empty
+	 * response rather than a 400, so a strict client schema still decodes it.
+	 */
+	public function test_get_data_accepts_retired_checklist_slug() {
+		wp_set_current_user( $this->admin_id );
+
+		$request = new WP_REST_Request( Requests::GET, '/wpcom/v2/launchpad' );
+		$request->set_param( 'checklist_slug', 'start-writing' );
+		$result = rest_do_request( $request );
+		$data   = $result->get_data();
+
+		$this->assertEquals( 200, $result->get_status() );
+		$this->assertSame( array(), $data['checklist'] );
+		// is_enabled is a required boolean in the client type, never null.
+		$this->assertFalse( $data['is_enabled'] );
+		$this->assertFalse( $data['is_dismissed'] );
+		$this->assertFalse( $data['is_dismissible'] );
+	}
+
+	/**
+	 * Retired slugs are read-only compatibility. Dismissing one is a mutation and
+	 * must be rejected, so no dismissal is persisted for a checklist that is gone.
+	 */
+	public function test_dismissing_a_retired_checklist_slug_is_rejected() {
+		wp_set_current_user( $this->admin_id );
+
+		$result = $this->call_launchpad_api(
+			Requests::POST,
+			array(
+				'is_checklist_dismissed' => array(
+					'slug'         => 'start-writing',
+					'is_dismissed' => true,
+				),
+			)
+		);
+
+		$this->assertEquals( 400, $result->get_status() );
+	}
+
 	/**
 	 * Test can_access.
 	 */
@@ -410,7 +451,7 @@ public function test_get_tasklist_using_goals( $site_goals, $enable_checklist_fo
 		update_option( 'site_goals', $site_goals );
 
 		$data = array(
-			'checklist_slug'             => 'start-writing', // This should get ignored, due to the use_goals flag.
+			'checklist_slug'             => 'design-first', // This should get ignored, due to the use_goals flag.
 			'launchpad_context'          => 'customer-home',
 			'use_goals'                  => true,
 			'enable_checklist_for_goals' => $enable_checklist_for_goals,