DevelTestController.php

<?php

namespace Drupal\devel_test\Controller;

use Drupal\Core\Controller\ControllerBase;

/**
 * Returns responses for devel module routes.
 */
class DevelTestController extends ControllerBase {
    
    /**
     * Returns a simple page output.
     *
     * @return array
     *   A render array.
     */
    public function simplePage() {
        return [
            '#markup' => $this->t('Simple page'),
        ];
    }

}