I have an on-premise client-server application, which uses Google API. Until now we were happy with out-of-band flow (redirect_uri=urn:ietf:wg:oauth:2.0:oob), with manual copy-paste of authentication code, but Google is going to block it soon.
The problem is that Google requires a fixed list of allowed redirect URIs, but I do not know internal addresses of application server inside the intranets. For desktop client I just use http://localhost/xxx, but I do not know what to do with web clients.
The only solution I see is to set up a public authentication server. The client will pre-register the state token and the actual redirect uri on the authentication server before calling Google authentication. But I am not sure this is a good idea.