Why not send the response as an event? The HTTP status code would be – event received. The event would be status updated.

Existing Wording:

For websocket subscriptions, the id of the event notification and the HTTP status code is communicated from the client to Hub through the existing websocket channel, wrapped in a json object. Since the websocket channel does not have a synchronous request/response, this id is necessary for the Hub to correlate the response to the correct notification.
Feedback from implementers is requested here. This is the only proposed communication from the subscriber to the Hub over websockets and the use of an HTTP status within a websocket connection, wrapped in json is weird. However, it seems important to enable the Hub to optionally track and/or broadcast synchronization state.