Is plan9 sufficiently unix to be on-topic here?
(I suppose it might not not really be a question of sufficiency; perhaps plan9 is too unix for this site?)
It has been my opinion for a while now that the “real” name of this site is “*nix.SE”, not “Unix.SE”. Since BSD, HURD, Mach and various other kernels (and their constituent operating systems) have seemed welcome here, I no longer see the usefulness in allowing only UNIX and Linux and close derivations thereupon.
Besides, even if we were to consider this as to only be on-topic for “unicies,” I would be amazed to see wide support for the notion that plan9 (and its currently maintained forks, à la 9atom and 9front) do not fall in that category. Not only was plan9 brought to bear by the same organization as UNIX, but has been regarded as having taken UNIX philosophy to its logical next evolution where UNIX itself faltered (read “Everything is a file”).
I am continually tempted to switch one of my machines over to 9front and see if I might find a home there; I see no compelling reason that *nix.SE should not become a home for plan9 as well. As always, I remain open to hearing well-reasoned counterarguments.
Although Plan9 and Unix are connected both history-wise and technology-wise, I don't think it really rates as a unix system. Then again modern unices have acquired features inspired from Plan9 (UTF-8,
/proc, FUSE, maybe even dbus, ...).
While I'm sympathetic to the wish to have a good place to discuss “alternative” operating systems, I don't want Unix.SE to be about all-but-Windows. This is primarily a political concern: I don't want to spread the idea that Windows is the normal operating system, and everything else is lumped together under as “alternative”, only-for-nerds, not-serious...
So if Plan9 is to be considered on topic (count me as mildly against), we need to draw a line somewhere (of course getting into the what's-a-unix debate). What about Hurd? Multics? ITS? QNX? VMS? CP/M? OS/2? Windows?