Any federated protocol can use an ipv6 mesh vpn (or any global vpn really) to become fully decentralized. I used SIP over Cjdns for a long time, using more XMPP and Matrix now. SMTP over Cjdns lets me contact old friends moved halfway around the world. Whether they are using amateur wifi links, DSL, or conventional (since 1996) ISP.

https://fedoramagazine.org/decentralize-common-fedora-apps-cjdns/