You can check middlewares (doc) and plugins (doc), but I would assume this is not possible, you don’t want Traefik to use resources for longer than necessary (I think that’s at least the approach of the developers). It kind of would provoke a DoS when more requests come in.
You could create a catchall router to a custom target to return an empty response with a custom error status code. Keeping the connection open would probably not work that way, as Traefik might timeout and send that error status back.