My understanding is that while ZFS might be a great fit on the desktop, but it's way too RAM hungry for phones/ipads. Maybe with Apple contribution it could work better on mobile? Maybe I'm just ignorant of different tuning or features that could be disabled? I've only used it in rather simple scenarios.
APFS was pretty explicitly designed for mobile, so I wouldn't be surprised if ZFS had been adopted on desktop they still made APFS for mobile.
Apple has historically be very stingy about RAM in iPhones. It looks like modern RaspberryPis today have 2, 4, or 8gb. The current-gen iPhone SE has 3gb, 11 standard/pro/max have 4gb. They're in the same ballpark. So it is possible, but Apple is so incredibly focused on mobile I can see them still investing in a custom filesystem focused on SSD, low memory overhead, per-file encryption, firmlinks, and whatever custom stuff they add for system updates, securing the boot volume, etc.
APFS was pretty explicitly designed for mobile, so I wouldn't be surprised if ZFS had been adopted on desktop they still made APFS for mobile.