Is it done on purpose?
After a search around I found some information states that zfs is broken on 21.10
Hmm, when upgrading from what?
The LXD snap package is the exact same on 21.04 and 21.10 so no upgrade actually occurs, LXD also has no way to convert from ZFS over to dir so I’m quite confused by what you’re describing 
Sorry about previous confusion. It seems that since zfs is not installed on the system, lxd will use dir pool.
ZFS tooling is bundled in the LXD snap. All that is required on the host system is a kernel that supports ZFS (which the Ubuntu kernels do).
After checking I think my previous information was wrong again. I use lxd init --auto and it will use dir pool anyway.
Yes that is correct. You can create a ZFS storage pool using lxc storage create <pool> zfs