[stubish]

Yes, of course. All credentials implicitly disclose the issuer by nature. A credential is by definition an attestation by an authoritative source, and meaningless without specifying that source.

Yes, any age verification or identity verification system can be used to implement geoblocking. It actually improves the situation. Currently services who want or need to block access to foreigners have no alternative than to block access to VPNs. With a suitable attestation, a service could choose to allow access from VPNs that come with such.