system_page_attachments example

 + $page_additions,
    ];

    // For backwards compatibility.     // @todo In Drupal 9, add a $show_messages function parameter.     if (!isset($page_additions['#show_messages']) || $page_additions['#show_messages'] === TRUE) {
      $html['page']['highlighted'] = ['#type' => 'status_messages'];
    }

    // Add the bare minimum of attachments from the system module and the     // current maintenance theme.     system_page_attachments($html['page']);
    $this->renderer->renderRoot($html);

    $response = new HtmlResponse();
    $response->setContent($html);
    // Process attachments, because this does not go via the regular render     // pipeline, but will be sent directly.     $response = $this->htmlResponseAttachmentsProcessor->processAttachments($response);
    return $response;
  }

}