Something that some security conscious folks care about.
Probably impractical for most server workloads (so not an alternative to Solaris on SPARC) but worth mentioning.
By the way Linux does it, but then again Oracle has its own distro.
https://www.oracle.com/servers/sparc
Linux while not being UNIX, does support it as Oracle upstreamed SPARC ADI support during the brief time they had Oracle Linux support for SPARC.
https://docs.kernel.org/arch/sparc/adi.html
Now why someone would use unsupported OpenBSD on SPARC for the kinds of clients that pay for this stuff, beats me.
Assuming it does even support SPARC ADI.
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
I'm sure Oracle milks commercial Solaris efficiently, but I imagine it's hard to find new customers for it.