I don't think any mainstream OS ever dies - instead, they often enter the hobbyist realm. Amiga OS and VMS are still actively developed this purpose (just not something you'd use in production). I imagine it's the same for OpenIndiana as Solaris had a large user base back in the day.
Their distribution is called Helios and is based on OmniOS. I think they were supposed to make it more suitable for consumer use / home install at some point - the package repository is public if you want to try to cobble it together on your own.
It's still private. I believe this is a case of "just gotta open it," but that takes some work (and I am not sure, I don't know all the details yet, I could be wrong). I have it on my to-do list to try and investigate this and get it done sometime soon, we'll see.
Anyone who cares about flexibility or quality? Those blobs can't be audited for bugs, can't get new features from the community, present a pain point for refactors, and I would expect impede porting to new architectures (though the existence of the ARM port makes me less sure of that point).
I think it doesn't have much support because the majority of SPARC gear out there runs better with SunOS. Solaris was a big jump up in processing requirements.
The improvements regarding memory tagging, the first UNIX architecture to ship memory tagging in production for years, has been done under Oracle management.
Any illumos distro will be better at ZFS than anything Linux-based. Granted, the amount will vary by distro; last I saw, Ubuntu had relatively good support. Beyond that, I'm personally quite fond of the overall design and polish of openindiana, at least. One example is how it automatically handles ZFS snapshots and boot environments with the package manager - all the pieces are there to do that on other systems, but setting up the integration is harder.
https://en.m.wikipedia.org/wiki/OpenIndiana