What is the need for the user ID in: https://api.stackexchange.com/docs/user-inbox
Given that it will only return the inbox of the authenticated user, and deny a mismatching user id.
The only needed route is the Network /users route ( https://api.stackexchange.com/docs/inbox) surely? As the inbox is a Network feature anyway, the site supplied is irrelevant (well judging by the docs and that both routes return the same content?)