AIX is still supported and sold, so quite current?
replySome that I used that are gone... Ultrix (MIPS), Clix, Irix, SunOS 4, SCO OpenServer, TI System V.
NeXTstep?
(Leaving aside fun spitballing about whether Tahoe is morally OPENSTEP 26, and whether it was NeXT that actually bought Apple for negative $400 million...)
replyAlas, I never had access to any of the Next environments, until PPC MacOS.
replyI did hold a copy in my hands for 486-class machines in the college bookstore.