robots.txt

// SiteController
public function actionRobots()
    {
        header("Content-type: text/plain");
        Yii::$app->response->data = DbText::widget(['key' => 'frontend.web.robots.txt']);
        Yii::$app->response->format = \yii\web\Response::FORMAT_RAW;
        return Yii::$app->response;
    }

// /frontend/config/_urlManager.php
['pattern' => 'robots', 'route' => 'site/robots', 'suffix' => 'txt'],