Drupal 8, make comment subject a required field
Here is how, you will need to create a custom module , and add this ... to yourmodule.module file ...

/**
 * Implements hook_form_alter().
 */
function yourmodule_form_alter(&amp;$form, \Drupal\Core\Form\FormStateInterface $form_state, $form_id)
{
  if ($form_id === 'comment_comment_form') {

    $form['subject']['widget'][0]['value']['#required'] = TRUE;

  }
}