Relays don't have to store in a db, be limited to one AZ, or serve over websockets. It would also be easy to support imgproxy syntax as part of the spec. A relay is just a server, anything a server can do a relay can too. The benefit is access to >1 image server with the same API.

That said, you're right that moderation (or at least operator willingness to take responsibility for what they're hosting) is a hard requirement. This is a much bigger ask than for regular relays, it's ok if we have big operators here IMO.