The following sentence that describes Bad_WaitingForInitialData is confusing:

"In such cases the server can optionally send a Notification with this status prior to the Notification with the first valid value."

The word "optionally" is confusing; does it refer to the notification or the status code? Does this mean that the server is NOT required to send an initial data-change if it does not have the data yet, but if it does then it can send zero/null values along with Bad_WaitingForInitialData.

I think the sentence could be rephrased:

"In such cases the server can send an initial data-change Notification with this status prior to the Notification with the first valid value."

I think it's important to communicate that the initial data-change is still sent with or without valid data.