I worked on Apple’s ZFS port, and it was indeed licensing. Apple’s lawyers took a closer look at the CDDL when we were close to shipping and found some provisions that were unacceptable to them.
It's a shame, because DTrace is cool and very powerful, but it seems almost nobody bothered to actually learn how to use it or cared about supporting it.
DTrace is definitely cool and powerful. I'm surprised to hear that few people at Apple cared about it - I'd have thought it was an essential tool for them to solve certain kinds of problems. It's funny, because the tool itself is relatively simple. It seems to me that most of the knowledge needed to use it effectively is an understanding of the code being instrumented, rather than of DTrace itself. Hopefully the sample scripts degrading does not foreshadow the probes themselves degrading too.
Trying to use DTrace with SIP also warns that "some features will not be available", though I don't know offhand what DTrace features don't work with SIP.
Edit: SIP is only partially disabled on my system, just enough to allow DTrace to work.
[1] https://web.archive.org/web/20160809042951/https://internals...
https://gnu.wildebeest.org/blog/mjw/2018/02/14/dtrace-for-li...
1) AFAICT they didn't relicense any of the ZFS code, just dtrace.
2) Even if they had, GPLv2 code still wouldn't be able to go into xnu.
3) Even if it could, Apple has long since moved on from ZFS. They've taken the time to write a new filesystem (APFS) already, so it's hard to see them having any interest in ZFS at this point (and yes, I know APFS doesn't have all the features of ZFS).