improvements-2.x-dev/modules/improvements_title/tests/modules/improvements_title_test/src/Controller/ImprovementsTitleTestController.php

<?php

namespace Drupal\improvements_title_test\Controller;

use Drupal\Core\Controller\ControllerBase;

class ImprovementsTitleTestController extends ControllerBase {

  public function alteredRouteTitle(): array {
    return [
      '#markup' => 'Hello, world!',
    ];
  }

}