Even more so: The 9P protocol https://en.wikipedia.org/wiki/9P_(protocol)

Maybe the most mainstream incarnation is its use in the Windows Subsystem for Linux (WSL).

It never left :p (I think that there are still active forks of Plan9 and Plan9 itself has definitely influenced some linux features or so I have heard)

Yes there is the great 9front and the useful plan9port. I run 9front on an old thinkpad, but I plan on doing a little more with it, having a dedicated CPU server, storage server etc will be the next step