The broad state of ZFS on Illumos, Linux, and FreeBSD (as I understand it)
Once upon a time, Sun developed ZFS and put it in Solaris, which was good for us. Then Sun open-sourced Solaris as 'OpenSolaris', including ZFS, although not under the GPL (a move that made people sad and Scott McNealy is on record as regretting). ZFS development continued in Solaris and thus in OpenSolaris until Oracle bought Sun and soon afterward closed Solaris source again (in 2010); while Oracle continued ZFS development in Oracle Solaris, we can ignore that. OpenSolaris was transmogrified into Illumos, and various Illumos distributions formed, such as OmniOS (which we used for our second generation of ZFS fileservers).
Well before Oracle closed Solaris, separate groups of people ported ZFS into FreeBSD and onto Linux, where the effort was known as "ZFS on Linux". Since the Linux kernel community felt that ZFS's license wasn't compatible with the kernel's license, ZoL was an entirely out of (kernel) tree effort, while FreeBSD was able to accept ZFS into their kernel tree (I believe all the way back in 2008). Both ZFS on Linux and FreeBSD took changes from OpenSolaris into their versions up until Oracle closed Solaris in 2010. After that, open source ZFS development split into three mostly separate strands.
(In theory OpenZFS was created in 2013. In practice I think OpenZFS at the time was not doing much beyond coordination of the three strands.)
Over time, a lot more people wanted to build machines using ZFS on top of FreeBSD or Linux (including us) than wanted to keep using Illumos distributions. Not only was Illumos a different environment, but Illumos and its distributions didn't see the level of developer activity that FreeBSD and Linux did, which resulted in driver support issues and other problems (cf). For ZFS, the consequence of this was that many more improvements to ZFS itself started happening in ZFS on Linux and in FreeBSD (I believe to a lesser extent) than were happening in Illumos or OpenZFS, the nominal upstream. Over time the split of effort between Linux and FreeBSD became an obvious problem and eventually people from both sides got together. This resulted in ZFS on Linux v2.0.0 becoming 'OpenZFS 2.0.0' in 2020 (see also the Wikipedia history) and also becoming portable to FreeBSD, where it became the FreeBSD kernel ZFS implementation in FreeBSD 13.0 (cf).
The current state of OpenZFS is that it's co-developed for both Linux and FreeBSD. The OpenZFS ZFS repository routinely has FreeBSD specific commits, and as far as I know OpenZFS's test suite is routinely run on a variety of FreeBSD machines as well as a variety of Linux ones. I'm not sure how OpenZFS work propagates into FreeBSD itself, but it does (some spelunking of the FreeBSD source repository suggests that there are periodic imports of the latest changes). On Linux, OpenZFS releases and development versions propagate to Linux distributions in various ways (some of them rather baroque), including people simply building their own packages from the OpenZFS repository.
Illumos continues to use and maintain its own version of ZFS, which it considers separate from OpenZFS. There is an incomplete Illumos project discussion on 'consuming' OpenZFS changes (via, also), but my impression is that very few changes move from OpenZFS to Illumos. My further impression is that there is basically no one on the OpenZFS side who is trying to push changes into Illumos; instead, OpenZFS people consider it up to Illumos to pull changes, and Illumos people aren't doing much of that for various reasons. At this point, if there's an attractive ZFS change in OpenZFS, the odds of it appearing in Illumos on a timely basis appear low (to put it one way).
(Some features have made it into Illumos, such as sequential scrubs and resilvers, which landed in issue 10405. This feature originated in what was then ZoL and was ported into Illumos.)
Even if Illumos increases the pace of importing features from OpenZFS, I don't ever expect it to be on the leading edge and I think that's fine. There have definitely been various OpenZFS features that needed some time before they became fully ready for stable production use (even after they appeared in releases). I think there's an ecological niche for a conservative ZFS that only takes solidly stable features, and that fits Illumos's general focus on stability.
PS: I'm out of touch with the Illumos world these days, so I may have mis-characterized the state of affairs there. If so, I welcome corrections and updates in the comments.