Sadly, they had to drop the whole thing because of the murky licensing on ZFS.
I sometimes dream of a world with a ZFS root partition on my MacBook Pro...
Not quite "it just works," but it's an option...
> When Apple included DTrace in Mac OS X a point in favor was that it could be yanked out should any sort of legal issue arise. Once user data hit ZFS it would take years to fully reverse the decision.
From the paragraph following your quote:
> Finally and perhaps most significantly, personal egos and NIH (not invented here) syndrome certainly played a part. [...] [C]ertain leads and managers preferred to build their own rather adopting external technology—even technology that was best of breed. They pitched their own project, an Apple project, that would bring modern filesystem technologies to Mac OS X.
and
> Licensing FUD was thrown into the mix; even today folks at Apple see the ZFS license as nefarious and toxic in some way whereas the DTrace license works just fine for them. Note that both use the same license with the same grants and same restrictions.
Leventhal's chronology continues to suggest that ZFS on OS X re-emerged even after this licensing argument had been advanced, and that the project was finally killed by Larry Ellison himself, in the interest of keeping his personal friendship with Steve Jobs unaffected by business considerations.
While it is of course possible that Apple is willing to accept any "murkiness" around the license as it pertains to DTrace but not willing to do so as it pertains to ZFS, it just doesn't seem like your original statement that "they had to drop the whole thing because of the murky licensing on ZFS" represents the situation clearly (at least not if we accept the version of events as told by Adam Leventhal; personally, I have no direct knowledge).
GPL and AGPL differ vastly in a few important aspects that make GPL pretty OK for commercial companies, and AGPL an absolute no-no.
Some prominent former Sun folks have publicly claimed the GPL incompatibility was intentional, other equally prominent folks have disagreed.
Proper Time Machine "v2" is a bit more complex (basically a rsync clone based on local and remote snapshots), but still way simpler than the current "v1" method, which builds its own change table from fsevents and has to cope with a lack of snapshots in HFS+.
They can't use APFS as a Time Machine "v1" store because the unique feature of HFS+ to support directory hard-links did not carry over to APFS. Migration of a v1 to v2 volume would be a fun exercise in translations (folder hierarchy collapsed to volume snapshots, and differing crypto approaches for encrypted backups).
I suspect they are trying to get APFS in an evolutionary "stable" place so they can publicly document it with the iOS 12/macOS Mojave/etc releases - the changes they are making to support HDDs and Fusion drives this year are likely a big internal evolution.
I also suspect Apple is just not in a rush to move time machine over - it already works, has a surprising amount of integration across the OS for backup/recovery, and people tend to value their backups working vs using the newest technology.
Eventually, my belief is they will roll out Time Machine v2 and leave v1 (on HFS+) only supported for restoration. Just like with v1 today, non-APFS volumes will get a sparse bundle disk image to hold the APFS data. But they will wait until they have a strategy to migrate the external volumes and disk images.