Benji Fisher
--- a/core/modules/block/src/Controller/BlockController.php
+++ b/core/modules/block/src/Controller/BlockController.php
+   * Provides a redirect and optionally adds warning messages.
+   *
+   * @param \Drupal\Core\Routing\RouteMatchInterface $route_match
+   *   The current route match. The route name should end in '.bc'.
+   * @param \Symfony\Component\HttpFoundation\Request $request
+   *   The current request.
+   * @param string $change_record
+   *   The URL of the change record, to be included in the log message.
+   * @param \Psr\Log\LoggerInterface|null $logger
+   *   (optional) A logger for the warning message.
+   * @param \Drupal\Core\Messenger\MessengerInterface|null $messenger
+   *   (optional) A messenger for the warning message.
+   *
+   * @return \Symfony\Component\HttpFoundation\RedirectResponse
+   */
+  protected function redirectWithWarning(RouteMatchInterface $route_match, Request $request, string $change_record, ?LoggerInterface $logger = NULL, ?MessengerInterface $messenger = NULL): RedirectResponse {
--- a/core/modules/block/tests/src/Functional/BlockAdminThemeTest.php
+++ b/core/modules/block/tests/src/Functional/BlockAdminThemeTest.php
+      $this->assertSession()
+        ->pageTextNotContains($old_path);
+    }
+  }
+
+  /**
+   * Provides data for testBlockPageRedirects.
+   */
+  public function providerTestBlockPageRedirects(): array {
+    return [
+      'block.admin_demo' => [
+        '/admin/structure/block/demo/{theme}',
+        '/admin/appearance/block/demo/{theme}',
+        FALSE,
+      ],
+      'entity.block.delete_form' => [
--- a/core/modules/block/block.links.menu.yml
+++ b/core/modules/block/block.links.menu.yml
 block.admin_display:
  title: 'Block layout'
-  parent: system.admin_structure
+  parent: system.themes_page
  description: 'Configure what block content appears in your site''s sidebars and other regions.'
  route_name: block.admin_display
+block.admin_display.bc:
+  title: 'Block layout'
+  parent: system.admin_structure
+  description: 'Use the link under Appearance.'