To be fair, when it comes to sockets, pretty much every OS' interface is identical, all taken from BSD.

Most OS also have platform specific extension over standard apis. This is the same category. They will not(and can not) deprecate BSD socket. Posix and BSD socket are totally different from OpenGL/CL. It's just an additional api for one who want to have maximum control.

I was replying to "Why does Apple have to make everything proprietary? It's kinda annoying."

And related to sockets, yeah when those OSes do provide some kind of POSIX like compatibility, and even then some advanced configurations are only possible via the OS specific APIs.

Microsoft has open sourced a ton of their code in the last few years

Just like Apple,

https://opensource.apple.com/

http://clang.llvm.org/

https://swift.org/