Depending on the usecase, rclone can expose an S3 endpoint via `rclone serve s3` to route to another protocol, eg sftp.

I mention it not to shill rsync.net, but to shill rclone, because when I discovered it I was even more impressed with it.

Obviously having to run a command and apply some amount of plumbing is different to a service just providing that API at the outset so the applicability for users will differ but still, rclone is very cool!