It's a shame that btrfs seems to be dead in the water. I guess nobody with the resources to build such a filesystem actually needs its snapshotting and integrity features at the filesystem level?
Just because Red Hat marked btrfs deprecated on their systems, doesn't mean the technology itself is deprecated.
Also, from what I've read, they deprecated it because of resources issues rather than tech issues (seems like they had 0 btrfs devs and lots of XFS specialists on the team).
SUSE is pretty much dead. Anything still running on that should be planning budget and resources for a migration.
For the record I would be very glad if I could seamlessy use ZFS, but from my perspective it looks like a lot of work that can break in unexpected places.
I used these instructions:
https://nixos.wiki/wiki/NixOS_on_ZFS
Ironically, I felt less of a need to use ZFS on NixOS. I don't have a large pool and snapshotting the system is not really necessary, since in NixOS you can always roll back to a non-GCed previous version on your system. But I use filesystem compression and might use snapshots on /home.