I like that but how do we keep censorship decentralized like SMTP?

You mean *unlike* SMTP? Signatures and relay hints

Easy, same way the experimental feature in element / adblocks do it. You create block lists people can subscribe to in their client regardless of which relay. Both blacklist and whitelist modes. That way people can tailor the experience tp their liking using lists they trust. If a list gets overly censorious you just replace it.