If you just put stuff on some standard storage (FTP, B2, etc.) without any permissions set up, then no backup client could stop the attacker from using the credentials you've deployed to the backup client to login to your storage and delete your backup files. This is not really specific to restic.
Accessing old files from a backup client is a weakness. Though, personally, I don't store things on a system that this system is not supposed to know, so if someone compromises this system and they see data from the past year... that's nearly the same as just seeing the data that is on this system today. Nevertheless, this could be solved by using public keys, so the backup clients has an encryption but no decryption key (of course it's not as trivial as this sounds, e.g. how could it still do deduplication etc.), but restic does not do this.