With Authenticated Web Messaging, can Genesys Cloud detect end-user’s identity attributes?

Yes, but only if the profile scope is set. At the moment the only OpenID Connect Claims that are automatically captured by Genesys Cloud are sub, given_name and family_name, which are then exposed as pre-built variables within Architect Inbound Message Flows as respectively addressDisplayable (sub claim) and name (concatenating given_name and family_name). For more information, see Authenticated web messaging attributes.