OAuth flow improvement with separate path for requests

In OAuth specification is talking about redirect_uri. This redirect_uri is used during Authorization Requests, such as Code Request or Access Token Request. Redirect_uri option should consists of registered URI of the client application, but not exactly the same. There could be set path for separate managing of Authorization, like URI/authorization, rest of code could be in main URI. The same is with logout URI/logout, URI/login etc.
This not work here. Setting separate path for specific request is comfortable.

OAuth2 will render this unusable.

Where ?? If it was render as unusable then for what is redirect_uri. URI is registered during client registration. redirect_uri is used during code request and access token request for what ??