I am interested in any answer to this

> If the request is something like /user/detail/12345 and user 12345 doesn't exist, what should the response be?

This is quite an interesting question. If we consider that webservers host paginated applications, the answer would be that this is indeed an application level concern and we should return a 404 since the resource is not found.

SPAs without SSR may have muddled this since the application is about serving a client side application then. We could expect a 200.

It is not as straightforward as it seems perhaps...