ResourceNotFoundController.php

<?php

/**
 * Class ResourceNotFoundController.
 */
class ResourceNotFoundController extends ControllerBase {

  /**
   * Display the Resource Not Found message.
   *
   * @return array
   *   Return page content.
   */
  public function index() {
    return [
      '#type' => 'markup',
      '#markup' => $this->t('Content located in (theme)/templates/page--resource-not-found.html.twig.')
    ];
  }

}