It has been deprecated since Windows 8, as mentioned on this answer https://superuser.com/a/495367/128124 with the following screenshot to back it:
Feature request: https://windows.uservoice.com/forums/265757-windows-feature-suggestions/suggestions/6573649-full-posix-support
According to an article on How-To Geek, it was removed from Windows 8.1.
On the server side, it was officially removed in Windows Server 2012 R2:
Subsystem for UNIX-based Applications
Windows Server 2012: Deprecated
Windows Server 2012 R2: Removed
Source: Features Removed or Deprecated in Windows Server 2012 R2
2016 Windows Subsystem for Linux
In 2016 a new official Linux-like API called "Windows Subsystem for Linux" was announced. It includes Linux system calls, ELF running, parts of the /proc filesystem, Bash, GCC, (TODO likely glibc?), apt-get and more: https://channel9.msdn.com/Events/Build/2016/P488 so I believe that it will allow Windows to run much, if not all, of POSIX. However, it is focused on developers / deployment instead of end users. In particular, there were no plans to allow access to the Windows GUI.