Note that LXD will assume it has full control over the ZFS pool or dataset. It is recommended to not maintain any non-LXD owned filesystem entities in a LXD zfs pool or dataset since LXD might delete them.
I don’t see anything that says that other backends like LVM or BTRFS aren’t subject to this limitation. I assume that’s not the case, but given how bad it would be if lxd started deleting other logical volumes I thought I’d check.
If it is safe to use lxd in existing lvm or btrfs volumes, I’ll take a stab at filing a docs clarification.