twig template suggestion for comment reply form
It's a bit unclear what exactly you want to achieve. But you can easily add another template suggestion based on the form ID by implementing hook_theme_suggestions_HOOK_alter for forms. The form ID can be found in the $variables then. But no matter if you are on the node route or the reply route this will simply give you comment_comment_form.

/**
 * Implements hook_theme_suggestions_HOOK_alter().
 */
function MYTHEME_theme_suggestions_form_alter(array &amp;$suggestions, array $variables) {
  $form_id = $variables['element']['#form_id'];
  // Add template suggestion based on form ID.
  $suggestions[] = 'form__' . $form_id;
}


If you want to do something different on the reply route you could check the current path for the /reply and /comment arguments OR check if the route parameters contain a PID, which would be the parent comment ID.

/**
 * Implements hook_theme_suggestions_HOOK_alter().
 */
function MYTHEME_theme_suggestions_form_alter(array &amp;$suggestions, array $variables) {
  // Add template suggestion based on form ID.
  $suggestions[] = 'form__' . $variables['element']['#form_id'];
  // Add template suggestion based on form ID and parent comment ID.
  if ($pid = \Drupal::routeMatch()-&gt;getParameter('pid')) {
    $suggestions[] = 'form__' . $variables['element']['#form_id'] . '__reply';
    $suggestions[] = 'form__' . $variables['element']['#form_id'] . '__reply__' . $pid;
  }
}


Which will give you the following theme suggestions.

&lt;!-- FILE NAME SUGGESTIONS:
   * form--comment-comment-form--reply--1.html.twig
   * form--comment-comment-form--reply.html.twig
   * form--comment-comment-form.html.twig
   x form.html.twig
--&gt;