I meant the/a Kosmos relay, but it could also be a more generic program that follows pubkeys on multiple relays. From an operator's point of view, it's not fun having to connect to arbitrary other relays via WebSockets, and you have to build in a lot of fault tolerance to deal with timeouts and such. Another option would be to do this directly in a Nostr client, of course, but then it wouldn't work with arbitrary clients. However, the RS-enabled client can sync all the events posted by other clients and back them up to RS once received.