https://www.ietf.org/rfcdiff?url1=draft-ietf-tcpm-rfc793bis-...
sure, maybe IETF uses their homegrown version, but - for example - TC39 uses GitHub https://github.com/tc39/ (here are the proposals for JS https://github.com/tc39/proposals ), whatWG also uses GH https://github.com/whatwg/html