Right so that is an LTS (Long Term Support) release and doesn’t get the newest features, so this will be in LXD 5 or you can switch to the latest/stable channel (But keep in mind you won’t be able to switch back as it will alter your database with latest schema).
See
Alternatively, if your not dead set on ZFS, have you looked at LVM?
In thin pool mode (the default for LXD), it supports efficient snapshots, and restoring from arbitrary snapshots, as well as copying from instances or instance snapshots, without preventing future restores or deletions.