[pjmlp]

They didn't had to copy anything, NeXTSTEP already did it by having a FreeBSD kernel copied into their hybrid kernel.

That doesn't mean it is going to stay like that forever, the UNIX network stack is already considered legacy for modern code.

https://developer.apple.com/videos/play/wwdc2017/707/

https://developer.apple.com/videos/play/wwdc2017/709/

Also I have some doubts about support in non-POSIX OSes like IBM i, z/OS, ClearPath MCP, or RTOS like INTEGRITY.