The idea behind introducing a new HTTP status code for blocking AI traffic should be to provide a clearer signal to both users and automated systems about the nature of the response. While a 404 indicates that a resource is not found, it doesn't convey the specific reason that the content is being restricted from AI access.
While User-Agents can easily be spoofed by users and automated traffic, as you mentioned in your earlier reply, the HTTP status code would be a clear message from the server.