I believe the request should be moved to the ideas section. At the moment what is possible to do via API is to periodically query the system (long-polling) and calculate the differences to understand what started and what ended. Obviously it is a viable, but costly, way also and above all for TeamViewer servers. I imagine that the time is ripe for a decision to implement a webhooks-based exchange system.
The notifications that I believe may be of interest to subscribers are: - start connections on channels connected to my license; - end of connection on channel connected to my license; - rejection of activation of new connection due to exhaustion of channels connected to my license.