[chasil]

AIX is still supported and sold, so quite current?

Some that I used that are gone... Ultrix (MIPS), Clix, Irix, SunOS 4, SCO OpenServer, TI System V.

https://en.wikipedia.org/wiki/Ultrix

https://en.wikipedia.org/wiki/Intergraph

[nxobject]

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...)

[chasil]

Alas, I never had access to any of the Next environments, until PPC MacOS.

I did hold a copy in my hands for 486-class machines in the college bookstore.