Decentralization works perfectly for forums. That's literally how the forums of the '90s worked: "Tom's Hardware" was never the only forum for Windows issues, "Codeguru" was never the only stop fo weird Visual Basic 5 error messages, etc.

Federation is orthogonal, and can cover a number of subjects - it's less about "what's out there" and more about "how do we get in touch". We have for example a sorta federated sign-in with things like OpenID. Lemmy, Mbin etc..., are built on the idea of federating the content. CDNs are literally federated caches.