moved to doing ZFS snapshots. There are probably some awkward ways to use delete access removal. Maybe some FTP server can do that. It also needs to block overwrites from attackers. Duplicati won’t overwrite backup files. New data gets uploaded, obsolete data gets compacted.
If you need a faster way than full chattr removal for maintenance, maybe folder sticky bit will do, combined with a chown of new files after upload. That can also prevent the file overwrite attack.
EDIT:
If filesystem support is not available, and you have extra space, you can clone the destination to another place not accessible from the client system and its attack. If you use rclone you can use
--backup-dir on your rclone sync to have both a clean sync and a way to cover delete attack, although the folder would also need cleaning occasionally to prevent it from growing without limit.
Reconstructing the original directory might get kind of painful, but at least the files would be there. Throwing in some dindex and dblock files that compact deleted might not bother a Direct restore.