Yep, S3 is reasonably easy to configure for immutability. I personally use restic to send (encrypted) blobs to https://www.borgbase.com which has append-only mode and monitoring to warn me if some backups didn't happen.
borgbase is another “little” service that I use and like just like tarsnap and to some extent rsync.net. And they also have an excellent gui app Vorta (it’s FOSS; borgbase dev is the maintainer).
