xyzulu
I just plonked the backup role on a spare empty VPS with single disk ext4. It didn't ask for a device, just a path/mountpoint. The mountpoint didn't even exist, it just created the directory at the mountpoint. And when moving that one site as test, it took up 17GB on the new Backup role VPS. So it works, but with zero deduplication.
I definitely hope that for new backup roles on servers with a dedicated block device, it will still format/mount it as btrfs and do the snapshot thing it did on V11, but I'm not rebuilding that backup server yet to find out.