How do I create a template for custom display mode of user registration form? I have multiple registration pages, each with corresponding to a form display under
admin/config/people/accounts/form-display. 
I added this to Mytheme.theme

function Mytheme_theme_suggestions_alter(array &amp;$suggestions, array $variables, $hook) {

  if ($hook == 'form' &amp; !empty($variables['element']['#id'])) {
    $suggestions[] = 'form__' . str_replace('-', '_', $variables['element']['#id']);
  }
}


Then I can create a template form--user-register-form.html.twig but that will affect all the form display modes. Is it a way to create a template for each display mode? For instance for a form display "Allied" something like form--user-registration-form---allied.html.twig ?
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());
    }
  }
}