How do I create a template for custom display mode of user registration form?
If you want to add template by Form mode try the following:

/**
 * Implements theme_theme_suggestions_alter().
 */
function YOURTHEME_theme_suggestions_alter(array &amp;$suggestions, array $variables, $hook) {
  if ($hook == 'form' &amp; !empty($variables['element']['#id'])) {
    /** @var \Drupal\Core\Entity\Entity\EntityFormDisplay $form_display */
    $form_display = $variables['element']['#process'][1][0] ?? NULL;
    if ($form_display) {
      $suggestions[] = 'form__' . str_replace('-', '_', $form_display-&gt;getMode());
    }
  }
}