Right, usually there isn’t anything between LVM and the filesystem.
I have one case where I have LUKS on top of LVM, but this is because I give LVs to a VM as its virtual disks, and the guest does LUKS on top – so neither system is fully aware of all of the layers. The host only knows about LVM and LUKS (but the LUKS container is not open), and the guest only knows about LUKS and the filesystem. Of course, restic wouldn’t be able to manage any snapshotting here because it can’t see the whole picture, so it’s not a great example.
Anyway, I think if restic is going to do any kind of automatic snapshots, it would make the most sense to at least start with filesystems that natively support snapshots (btrfs and ZFS) as they are the most straightforward. Restic doesn’t have to know or care about any storage layers whatsoever to make this work.
Supporting LVM snapshots will require probing the block storage layout. This is more complicated, harder to get right, and in the end restic could probably only support the simplest of layouts anyway.