Drupal 8 - add theme suggestions with form ids

By default, there is only one theme form suggestion for every form. To change this use this snippet inside your .theme file:

function scriptun_theme_suggestions_form_alter(array &$suggestions, array $variables){
    $formId = $variables['element']['#id'] ?? null;
    if ($formId) {
        $suggestions[] = 'form__' . str_replace('-', '_', $formId);
    }
}

Change scriptun with your theme name, and check suggestions. Example debug for login form:

<!-- THEME DEBUG -->
<!-- THEME HOOK: 'form' -->
<!-- FILE NAME SUGGESTIONS:
   * form--user-login-form.html.twig
   x form.html.twig
-->

You can remove -form suffix if you want:

function scriptun_theme_suggestions_form_alter(array &$suggestions, array $variables){
    $formId = $variables['element']['#id'] ?? null;
    if ($formId) {
        $suggestions[] = 'form__' . str_replace(['-', '_form'], ['_', ''], $formId);
    }
}

Suggestions should work anyway if you use dashes instead of underscores, but it's nice to keep Drupal naming convention