To identify which id or topic a notification is for you could always use different callbacks. For example yourapp.domain/api/streams/UserID, yourapp.domain/api/streams/AnotherUserID, or yourapp.domain/api/follows/UserID etc… Then you can still use the same handler for all of them, but easily be able to see from the request URL both the topic and userid that the notification is for.
I’m not sure why you’re not getting stream down notifications though, from my testing I’ve been getting the empty array to indicate the stream is offline, much like you would get from polling the API.