In order for ZFS send to be able to provide the same features as restic it would need to output a representation of the zfs merkle tree as a flat filesystem (but encrypted) -- which would allow a dumb server to deduplicate the tree (and ZFS is clever enough to already know what blobs exist on the remote side). I guess this was not done because a ZFS send stream might be more efficient for transfer (as you said). But this means that it's main use as a backup system requires having a ZFS server on the other end (in order to be efficient and useful as a backup store).

Again, I'm not bashing ZFS. My whole point is that restic is a neat and interesting project specifically because it doesn't require a clever server to provide its features -- that doesn't mean ZFS isn't a great project (far from it).

I use ZFS on my servers and love it, and I use restic for backups.