list/consult Taxonomy only for the respective author/creator in dashbboard

The WP_Term_Query class doesn’t have a method named set, and calling $query->set() in my case resulted in a fatal/critical error saying “Call to undefined method WP_Term_Query::set()“. :/

But you can directly modify the query_vars property (i.e. $query->query_vars), like so, to set, unset or override any query variables like meta_query:

// Use this:
$query->query_vars['meta_query'] = $meta_query;

// NOT this (which causes a fatal error):
$query->set( 'meta_query', $meta_query );

Or you could instead use the get_terms_args hook:

add_filter( 'get_terms_args', 'filter_cat_by_u_id' );
function filter_cat_by_u_id( $args ) {
    global $pagenow;

    if ( is_admin() && 'edit-tags.php' === $pagenow ) {
        $args['meta_query'] = array( your code here );
    }

    return $args;
}