I have some knowledge of the BSDs and they all are but OpenBSD is particularly belligerent about Licenses and they consider ZFS "non–free" on par with the GPL(I know I know) which is a huge barrier to entry into the kernel. They only begrudgingly accepted clang after they changed their license because there was no real alternative. Before that they had been using some ancient GPLv2 gcc.

Truth is, I've never had a catastrophic data loss on traditional filesystems like ext2 and ufs and the devs probably haven't had one either so they don't feel like the situation is the same as with lacking a compiler.

I read the article about why they removed soft updates and the gist of it was that the maintainer moved on and no one else was willing to understand the complex system anymore.

I'd imagine ZFS is at least on the same level of complexity.

So, two pretty tall hurdles to clear.

If someone gave them a filesystem with the right license, ideas from 20+ years ago and a promise to maintain it on OpenBSD forever, I'm sure they'd take it.