https://w3id.org/fep/844e should work:
{ "@context": [ "https://www.w3.org/ns/activitystreams", "https://w3id.org/fep/844e" ], "type": "Application", "id": "https://social.example/server", "implements": [ { "href": "https://datatracker.ietf.org/doc/html/rfc9421", "name": "RFC-9421: HTTP Message Signatures" } ] }