Omit custom post type from wp-sitemap.xml based on meta key using wp_sitemaps_posts_query_args

The error happens because the filter callback (shapeSpace_disable_sitemap_post_meta()) expects to receive two parameters ($args and $post_type) and yet you call add_filter() without setting the fourth parameter to 2.

So to solve the problem:

// Replace this:
add_filter('wp_sitemaps_posts_query_args', 'shapeSpace_disable_sitemap_post_meta');

// with this one:
add_filter('wp_sitemaps_posts_query_args', 'shapeSpace_disable_sitemap_post_meta', 10, 2);