For relays, that is more or less what the pub servers do. I connect to a relay, and if I subscribe to a channel or follow an individual, I end up with messages

For relays, that is more or less what the pub servers do. I connect to a relay, and if I subscribe to a channel or follow an individual, I end up with messages from months ago. The pub servers "gossip" with each other, so any particular pub server you connect to, should be able to catch you up on all of your friends and channels "gossip".