“For append-only the current idea is that this should not be done within borg, but solved by a missing repo object delete permission enforced by the storage.”
Append-only mode was borg’s defense against a compromised client wiping its own backups. Borg 2 removes it and tells users to set up no-delete credentials on the storage side. That works if your storage supports it and you configure it correctly. Ransomware only needs you to get it wrong once.