Wordpress taxonomy menu (wp_get_nav_menu_items hook)

functions.php

add_filter("wp_get_nav_menu_items", function ($items, $menu, $args) {
  $menu_id = 21; // Wordpress Menu ID
  $menu_item_id = 48; // Wordpress Menu Item ID
  $taxonomy_name = 'product_cat'; // Wordpress Taxonomy Code
  $max_depth = 3; // Maximum level of menu depth
  if( $menu->term_id != $menu_id  || is_admin()) return $items; 
  $get_nav_from_terms = function (&$items, $terms, $parent_menu_id, $order, $depth = 1) use ($max_depth, &$get_nav_from_terms) {
    if ($depth > $max_depth) return;
    foreach ($terms as $term) {
      $item = new stdClass();
      $item->ID = 1000000 + $order + $parent_menu_id;
      $item->db_id = $item->ID;
      $item->title = $term->name;
      $item->url = get_term_link($term);
      $item->menu_order = $order++;
      $item->menu_item_parent = $parent_menu_id;
      $items[] = $item;
      $terms_child = get_terms(array('taxonomy' => 'product_cat', 'parent' => $term->term_id ));
      if (!empty($terms_child)) {
        $get_nav_from_terms($items, $terms_child, $item->ID, $order, $depth + 1);
      }
    }
  };
  $terms = get_terms(array('taxonomy' => $taxonomy_name, 'parent' => 0));
  $get_nav_from_terms($items, $terms, $menu_item_id, 100);
  return $items;
}, 10, 3);