Created zsys snapshot and booted into it successfully.
Downgraded all zfs packages back to 2.0.6-1ubuntu2, to check that I can reproduce original failure. trying to boot test-sru-2 created with original packages resulted in booting to emergency mode.
Installed encrypted zsys impish system, enabled proposed, and applied all updates.
$ dpkg-query -W zfs-initramfs
zfs-initramfs 2.0.6-1ubuntu2.1
Created zsys snapshot and booted into it successfully.
Downgraded all zfs packages back to 2.0.6-1ubuntu2, to check that I can reproduce original failure. trying to boot test-sru-2 created with original packages resulted in booting to emergency mode.