restic’s rest-server append-only mode unfortunately doesn’t prevent data deletion under normal usage. More here: https://restic.readthedocs.io/en/stable/060_forget.html#secu.... Their workaround is pretty weak, in my opinion: a compromised client can still delete all your historic backups, and you’re on a tight timeline to notice and fix it before they can delete the rest of your backups, too.
That article says that a compromised client can not delete your historic backups, however, a compromised client could create enough garbage backups that an automatic job by an non-compromised administration account could delete them due to retention policies.
I'm not sure what exactly you expect that would be different?
