They've had a POSIX subsystem (SUA) for a while, but it's kind of ancient. I assume they resurrected it and implemented some Linux-specific APIs, which would actually be SO COOL.
The Windows POSIX subsystem which shipped in NT 3.5.1 was a minimal implementation of POSIX syscall API plus a userland toolset. That was replaced with Interix which was renamed Services for Unix (SFU) which had a more comprehensive kernel implementation and more up to date userland. However that tech was not resurrected to build the Windows Subsystem for Linux (WSL).
Importantly, WSL doesn't ship with a distro - we download a genuine Ubuntu userland image at install-time and then run binaries within it.
So can we use WSL by itself and pick a different distro, if we'd rather use say Alpine or openSUSE or Arch's userland?
I, for one, would like to see it resurrected. It's exceedingly useful, and is one major reason that I am not, nor will be, using Windows 10. This new subsystem does not have the things that I use SFU/SFUA for. Nor does it have the BSD-style toolset of SFU/SFUA.