The code below does work, just make sure you are editing the correct themes function.php file!
// Remove specific pages
function gt_disable_sitemap_specific_page($args, $post_type) {
if ('page' !== $post_type) return $args;
$args['post__not_in'] = isset($args['post__not_in']) ? $args['post__not_in'] : array();
$args['post__not_in'][] = 221; //locations
$args['post__not_in'][] = 261; //blog
return $args;
}
add_filter('wp_sitemaps_posts_query_args', 'gt_disable_sitemap_specific_page', 10, 2);