DeprecatedTwigTemplateController.php

<?php

namespace Drupal\deprecated_twig_template\Controller;

use Drupal\Core\Controller\ControllerBase;

/**
 * Controller for calling a deprecated theme.
 */
class DeprecatedTwigTemplateController extends ControllerBase {
  
  /**
   * Display the deprecated template with a message.
   *
   * @return array
   *   Render array containing the deprecated template.
   */
  public function deprecatedTwigTemplate() {
    return [
      '#theme' => 'deprecated_template',
      '#message' => 'This is a deprecated template. Use an alternative template.',
    ];
  }

}