It's unsupported though. As you mentioned, minor version releases and security patches certainly don't get updated in it. I couldn't find os_unfair_lock_with_options in it (maybe I just missed it). AFAIK the only reason you can still build from the source is because of the work from volunteers.

So, unless your target is Darwin, you can't be sure that the source is what your code is going to be interacting with. It's certainly a helpful starting point, but you do have to attempt to reverse engineer the interface to make sure you understand how it behaves.