Yes, that is limited to S3. But we will make it extensible and flexible so it will indeed respect the 304 Not Modified HTTP response and the Expiry date header. I think that s3fs already does that.
What we are going to do in addition to s3fs is to also offer the cache folder with read/write access so you can manually delete files if necessary.
There are few problems tho… The server holding the files (origin) needs to also export the list of files (folder listing). Otherwise, nobody knows if that file will be there or not