I’ve begun to wonder about this question. There’s nothing implied in the existence of the fediverse that it will include every site, and there’s nothing implied in the existence of every non-fediverse site which would stop them from forming their own fediverse. One could technically say email providers have already achieved this, though this only really applies to email providers which doesn’t make it as useful as the fediverse we know and love. Are there actually other fediverses out there?
Long ago Usenet and BBS networks worked in a manner that we could describe as federated, if you mean a decentralized system where servers could communicate with each other. I saw something a few months ago about a modern service that sounds kind of similar, but I don’t remember the name now. It seemed interesting but I put it in the back burner and then lost it.