Cleaned up basic customization below. The $_REQUEST['post_id']
is the same as the $post_id
provided in the arguments.
function custom_relationship_query( $args, $field, $post_id ) {
$args['tax_query'] = [
[
'taxonomy' => 'custom_taxonomy_name',
'field' => 'term_id',
'terms' => str_replace( 'term_', '', $post_id ),
],
];
return $args;
}
add_filter('acf/fields/relationship/query', 'custom_relationship_query', 10, 3);