All our production stuff was being deployed on Aix, HP-UX, Solaris and Windows NT/2000 Server.
Likewise most of my university degree used DG/UX and Solaris, when Red-Hat Linux was first deployed on the labs, it was after the DG/UX server died, and I was already on the fourth year of a five year degree.
We did use NT/2K internally but that was because we had some who insisted on using SMB via Windows.
Such fun times. The nix and nix-like OSes were spreading like fire. I never would have thought I'd ever wrangle them for the majority of my career.
Just because things hung around didn't mean that Sun/Solaris/Java were long for this world. Linux/x86 was just too cheap compared to SPARC gear. Even if it wasn't as robust as the Sun gear, it just made too much sense especially if you didn't have any legacy baggage.
But the x86 I was referring to in my comment above, Stratus, was (maybe still is?) an exotic attempt to enter the mainframe-reliability space with windows. IIRC it effectively ran two redundant x86 machines in lockstep, keeping them in sync somehow, so that if hardware on one died the other could continue. I have no idea how big their market was, but I know of at least one acquirer/issuer credit card system that ran on that hardware around 2002-3.