Adding a custom taxonomy to "nav_menu_item"

Question

Does anyone know if there is a way to handle a custom taxonomy for the nav_menu_item built'in post type? When I assign my custom taxonomy to this post type, i don't see any change in the menu editor... It works well with posts, pages and custom post types.

Here how I've registered my custom taxonomy:

  add_action( 'init', 'my_region_taxo', 0 );
  function my_region_taxo() {
    $labels = array(
        'name'                       => _x( 'Regions', 'Taxonomy General Name', 'my-text-domain' ),
        'singular_name'              => _x( 'Region', 'Taxonomy Singular Name', 'my-text-domain' ),
        (...)
    );
    $rewrite = array(
        'slug'                       => 'region',
        'with_front'                 => true,
        'hierarchical'               => true,
    );
    $args = array(
        'labels'                     => $labels,
        'hierarchical'               => true,
        'public'                     => true,
        'show_ui'                    => true,
        'show_admin_column'          => true,
        'show_in_nav_menus'          => false,
        'show_tagcloud'              => false,
        'rewrite'                    => $rewrite,
        'show_in_rest'               => true,
    );
    register_taxonomy(
        'my_region_taxonomy',
        array(
            'post',
            'page',
            'nav_menu_item'
        ),
        $args
    );
  }

Answer 1

This is not how you enable adding your taxonomy terms to menus. To do that you just set show_in_nav_menus to true when registering the taxonomy. You have it set to false.

$args = array(
    'labels'                     => $labels,
    'hierarchical'               => true,
    'public'                     => true,
    'show_ui'                    => true,
    'show_admin_column'          => true,
    'show_in_nav_menus'          => true, // This
    'show_tagcloud'              => false,
    'rewrite'                    => $rewrite,
    'show_in_rest'               => true,
);

register_taxonomy(
    'my_region_taxonomy',
    array(
        'post',
        'page', // Not here
    ),
    $args
);

Your taxonomy should not actually be a taxonomy for the nav_menu_item post type.

Answer 2

A quick way is to use the normal post edit page by enabling show_ui for the nav_menu_item post type.

add_filter('register_post_type_args', 'register_post_type_args', 10, 2);

function register_post_type_args( $args, $post_type ) {
    if ( $post_type === 'nav_menu_item' ) {
        $args['show_ui'] = true;
    }
    return $args;
}

Then you can inspect the menu to get the nav_menu_item post id, and replace it in the post edit url. I know it's not as good as being able to set the terms from the menu editor, but it's a quick way to get it working.