Mandatory WordPress for custom post type and all post as well as page. Remove if ( 'custom-post-type-slug' != $data['post_type'] ) { } else { } for all excerpt mandatory.

mandatory excerpt wordpress

function mandatory_excerpt($data) {
 //for multiple custom post type just add  && 'custom-post-type-slug' != $data['post_type']
   if ( 'custom-post-type-slug' != $data['post_type'] ) {
   } else {

$excerpt = $data['post_excerpt'];
  if (empty($excerpt)) {
    if ($data['post_status'] === 'publish') {
      add_filter('redirect_post_location', 'excerpt_error_message_redirect', '99');
    }
    $data['post_status'] = 'draft';
  }
   }
  return $data;
}
add_filter('wp_insert_post_data', 'mandatory_excerpt');
function excerpt_error_message_redirect($location) {
  remove_filter('redirect_post_location', __FILTER__, '99');
  return add_query_arg('excerpt_required', 1, $location);
}
function excerpt_admin_notice() {
  if (!isset($_GET['excerpt_required'])) return;
  switch (absint($_GET['excerpt_required'])) {
    case 1:
      $message = 'Excerpt is required to publish a post.';
      break;
    default:
      $message = 'Unexpected error';
  }
  echo '<div id="notice" class="error"><p>' . $message . '</p></div>';
}
add_action('admin_notices', 'excerpt_admin_notice');