@MikeP 

Thank you, but your comment doesn't answer the question.  The UDP port 9999 still seems to exist, but either it is looking for a new message to respond to, or it no longer actually listens to the open port.  I was looking for either the new message format, or confirmation of a 'deaf' listener.  I.e. Has TP-Link removed or changed prior functionality?

Your suggestion of a fixed address doesn't address the need for discovery.

As for 'aftermarket app', I am using a trivial application I wrote that meets my needs, and not a heavy weight 'home automation' application, nor the Kasa Android/Cloud based tool.