E.g. Amazon S3 and Backblaze B2 support this.

Re ransomware, how do you prevent it from deleting remote backups?

It can't, locks on objects cannot be removed in compliance mode. They can only expire.

https://www.backblaze.com/b2/docs/file_lock.html

https://docs.aws.amazon.com/AmazonS3/latest/userguide/object...