(This is just a very rough draft, I am only looking for conceptual feedback right now, so this is the bare minimum code to understand what it should do. I will add tests and fix docs when the conceptual review hurdle is cleared.)
This is an alternative approach to what was already attempted with #25690 and #24835. Please have a look there for more background information.
The -allowinbound option lets users explicitly mark networks as available for inbound connections, even if that network may be explicitly deactivated for outbound connections or if it is blocked for other reasons that are out of the user’s control. The effect is that the -externalip addresses of that network may be announced and the -getnetworkinfo RPC will show these addresses. These several users seem to have run into these issues, see for example #25669.