Fwiw, AIX and to a far lesser extent Solaris still exist. I'm not exactly sure why people are using them (AIX I can maybe understand because "no one got fired for buying IBM" or whatever but there really isn't any excuse to be running Solaris nowadays since ZFS runs on Linux and and 2 of the BSD based systems and oracle seems desperate to let it die)
So wait, sparc solaris is the only production unix with hardware memory tagging but also linux has it? Are we talking strict SUS compliant systems (current or former because for some reason solaris is no longer listed as such despite ostensibly still being compliant unless the SRUs have seriously FUBARed some things) or unices in general? because I'd argue anyone running SUS compliant systems out of anything other than their choice happening to be compliant is arguably even more niche than running AIX or Solaris for anything else
Why would someone use unsupported OpenBSD on SPARC for the clients that pay for it? Probably the same reason so many servers run on Rocky or Alma instead of RHEL, money. Perhaps they bought the hardware without the support contract for Solaris, or they don't want to keep paying for it
