Interix which later became, SFU (Windows Services for UNIX) and then finally SUA (Subsystem for Unix-based Applications) was initially developed in the 90's by Softway Systems, as an optional addon subsystem to the Windows family of OS's.

For the record, it did not ship built-in or as part of the kernel. Even within Microsoft, it seemed the project was in perpetual life-support-mode.

Still, I do feel a small pang of sadness. There was once a time in the early 2000's, where I would have loved to see this chimeral oddity succeed.

[1] https://en.wikipedia.org/wiki/Interix

And we now have the Windows Subsystem for Linux, which I believe is supposed to be entirely new, but carries on the strange saga of Windows and POSIX userspace coming together.