HP-UX is kind of an interesting tale in that it's always been underrepresented in the enthusiast community, perhaps because HPE never tried to put up hpux on intanium as an alternative to Linux on proliant, or maybe because it's core market seems to have been in the manufacturing sector where it supplanted hp3000 minicomputers and ran boring logistics software and factory control systems and never really made it big in education, research nor web hosting.

by 2004 HPUX was actually pulling off an successful migration from PA-Risc to Itanium and it lived on profitable for over a decade after that until HPE finally published an Roadmap for when they were going to end HPUX development which is currently scheduled to happen around 2025/26 with the last new itanium systems sold around 2018.

Irix was killed off by windows around the time the pc industry up with SGI's graphics capabilities. And i cant recall what happened to digital's true64(but it never really survived the merger of DEC into first Compaq and then HP).