Is mature Multisite

The current WP_Site object is saved as global $current_blog, so you can use

global $current_blog;
if ( isset( $current_blog ) && $current_blog->mature ) { ... }

There’s no convenience helper method to fetch this as there is get_current_site() for $current_site; there is also get_site() which you could use get_site()->mature but that doesn’t use the already-loaded WP_Site object.

I’d probably store the ‘premium’ flag in an option, though, which can be per-blog or per-site, but it depends how you want to use it e.g. I doubt you can search for blogs with a given option as easy as you can search for the ‘mature’ flag.