maintenance-1.0.0-beta1/src/Routing/MaintenanceRouteSubscriber.php

<?php

namespace Drupal\maintenance\Routing;

use Drupal\Core\Routing\RouteSubscriberBase;
use Drupal\maintenance\Form\MaintenanceModeForm;
use Symfony\Component\Routing\RouteCollection;

/**
 * Alters the core maintenance configuration form route.
 *
 * This allows the Maintenance module to provide advanced options
 * via its custom form instead of the default core form.
 */
class MaintenanceRouteSubscriber extends RouteSubscriberBase {

  /**
   * {@inheritdoc}
   */
  protected function alterRoutes(RouteCollection $collection): void {
    if ($route = $collection->get('system.site_maintenance_mode')) {
      // Override the route to use the module's custom form class.
      $route->setDefault('_form', MaintenanceModeForm::class);
    }
  }

}