It still is, but at this point I'm not sure why anyone would pick it over Linux for something new. All the killer Solaris features (ZFS, dtrace, zones, SMF) have good enough Linux equivalents (OpenZFS, eBPF, containers, systemd) and I'm not aware of any usecases where illumos outperforms Linux anymore.

OpenIndiana also has the problem that every commercial illumos user is using it for some niche purpose (networking infrastructure, storage appliance, that sort of thing) so it's basically up to a few unpaid volunteers working in their free time to adapt it for general desktop use. I'm not sure what the state of stuff like audio support or accelerated graphics looks like if you're on modern hardware.