Why would that not be possible? Especially since you say borg can do it, and borg is said to be incremental and deduplicated? Note that write-only or append-on

Why would that not be possible? Especially since you say borg can do it, and borg is said to be incremental and deduplicated? Note that write-only or append-only is a bit of a misnomer, since reading the files is fine (you need the decryption key anyhow before they're of any use). It's about not being able to overwrite or remove backup data without some verification that you're not ransomware or similar.