I understand that Android isn't a fork of Linux, I meant to imply that although Android is argued to be a Linux distro

Linux distros are not only "Linux", they are GNU/Linux. Linux is only the kernel of a GNU/Linux system (a distro). Android is not a distro because they don't have GNU, but Android is "Linux" because it has Linux as kernel.

I understand that Plan 9 is not being developed anymore,

It is not developed anymore in Bell Labs, but many of the original developers are part of the Plan9 Foundation and they are promoting it in different ways: https://plan9foundation.org/activities.html So I would not say that it "is not being developed anymore".

Antonio B.