REST filters only firing when I have a param set

If you want a parameter to be required for a post type’s REST API endpoint you can use the rest_{$this->post_type}_collection_params filter to filter the $args of the GET endpoint:

add_filter(
    'rest_car_collection_params',
    function( array $query_params ) {
        $query_params['visible_to']['required'] = true;
        
        return $query_params;
    }
);