If you accidentally reformat the ZFS drive, that will delete all your files and snapshots and then Syncthing will replicate those deletions.

I'm really not super familiar with Syncthing, but it sounds like it's niche is availability as opposed to durability.

Even if you format your drive and Syncthing copies over the empty files you will still have a history of snapshots on the other host running ZFS snapshots.

What if you accidentally format the drive on the host with the ZFS snapshots? Won't that result in synchronizing the deletion of those files to all your other devices?

Yes, for his setup (if I understand right). I'm suggesting having two hosts with ZFS snapshots running (independent of each other) with Syncthing between. So if you delete the files on one host and they get Syncthing'd across there will still be a ZFS snapshot history on the other host. I'd also have a cold storage backup on hand that is also ZFS. Having your backup as a regular filesystem is a very nice feature once it comes to recovery.

syncthing has send-only and receive-only folders. You can set a receive-only folder on your backup device and send-only on your phone or whatever. With incremental backups it's not an issue. Syncthing is just the tool to get your data to the place it needs to be. Just like rsync or cp

Anyone taking backups seriously already knows the 3-2-1 rule anyway.

Syncthing frequently stops syncing on my phone and requires me to delete everything and resync. I'm using receive-only folders. I think I'm an exception but it's been a pain in the ass.

If you accidentally delete your data, your data will be deleted.

This is the curse of sharp tools. Don't run unix if you don't want rm to rm.

I still also have backups, and zfs snapshots on a different independent machine.