fwiw keybase supports simple and easy signatures (and less verbose than PGP.) combined with an (imo) much more rational and discoverable attestation of identity, I think it's nearly ready to supplant PGP for this use case. email integration for signatures would indeed be nice.
Mind you, unauthenticated encryption in terms of signal does not mean that anyone can forge a message, rather that you can't prove to a 3rd party that this was a message sent by the sender rather than forged by you.
PGP and S/MIME do. The latter is used for all sorts of official messages for exactly that reason.
Unauthenticated encryption is rather worthless. Repudiation is a separate matter and threat model.