code: public function onKernelController(FilterControllerEvent $event)
      {
          if ($event->getRequestType() !== HttpKernelInterface::MASTER_REQUEST) {
              return;
          }
  
          if (!$this->isAdminSiteAccess) {
              return;
          }
  
          $currentRoute = $event->getRequest()->attributes->get('_route');
  
          foreach ($this->legacyRoutes as $legacyRoute) {
              if (stripos($currentRoute, $legacyRoute) === 0) {
                  $event->getRequest()->attributes->set('_controller', 'ezpublish_legacy.controller:indexAction');
                  $event->setController($this->controllerResolver->getController($event->getRequest()));
  
                  return;
              }
          }
      }
docs: Redirects configured routes to eZ legacy.
  
  @param \Symfony\Component\HttpKernel\Event\FilterControllerEvent $event