public static function userDeleteCleanInit(\XF\Service\User\DeleteCleanUp $deleteService, array &$deletes)
    {
        $deletes['xf_your_table_name'] = ['user_id = ?'];
    }
// or if more than one user_id is stored in your table
    
    public static function userDeleteCleanInit(\XF\Service\User\DeleteCleanUp $deleteService, array &$deletes)
    {
        $deletes['xf_your_table_name'] = ['user_id = ?', 'other_user_id = ?'];
    }