Redirect pre-13.7 Site Editor routes to site.editor.php

lib/compat/wordpress-6.0/edit-site-routes-backwards-compat.php

@@ -0,0 +1,33 @@
+<?php
+/**
+ * Whitelists the `theme.php` route and redirects the following routes:
+ * - `themes.php?page=gutenberg-edit-site`
+ * - `admin.php?page=gutenberg-edit-site`
+ *
+ * To `site-editor.php`.
+ *
+ * The old routes have been deprecated and removed in Gutenberg 13.7.0, but third-party
+ * consumer code might still be referencing them. In order to not break the Site Editor
+ * flows, we don't fully remove the old routes, but redirect them to the new one.
+ */
+
+ /**
+	* "Whitelists" the old routes. Without this, trying to access the old Site Editor
+	* routes result in a HTTP 403 error.
+  *
+	* The whitelist is done by adding an wp-admin submenu page that won't be rendered.
+  */
+function gutenberg_site_editor_menu() {
+	if ( wp_is_block_theme() ) {
+		add_submenu_page( 'themes.php', null, null, 'edit_theme_options', 'gutenberg-edit-site', 'gutenberg_edit_site_page' );
+	}
+}
+add_action( 'admin_menu', 'gutenberg_site_editor_menu', 9 );
+
+/**
+ * Does the actual redirect to the new route upon triggering of the `load-appearance_page_gutenberg-edit-site` action.
+ */
+function gutenberg_redirect_deprecated_to_new_site_editor_page() {
+		wp_safe_redirect( 'site-editor.php' );
+}
+add_action( 'load-appearance_page_gutenberg-edit-site', 'gutenberg_redirect_deprecated_to_new_site_editor_page' );

lib/load.php

@@ -74,6 +74,7 @@ function gutenberg_is_experiment_enabled( $name ) {
 require __DIR__ . '/compat/wordpress-6.0/edit-form-blocks.php';
 require __DIR__ . '/compat/wordpress-6.0/block-patterns-update.php';
 require __DIR__ . '/compat/wordpress-6.0/client-assets.php';
+require __DIR__ . '/compat/wordpress-6.0/edit-site-routes-backwards-compat.php';
 
 // WordPress 6.1 compat.
 require __DIR__ . '/compat/wordpress-6.1/blocks.php';