Alternative to new_to_publish Hook for Custom Statuses

new_to_holding doesn’t exist as a post status transition – you need to use the generic transition_post_status action. Something like (untested):

add_action('transition_post_status','my_holding_function', 10, 3);

        function my_holding_function( $new_status, $old_status, $post ) {
            if ( 'holding' == $new_status ) {
                // do stuff
            }
        }