If you’re looking for just a client plugin it does seem they’re all out-of-date:
https://github.com/tedward1/ts3-notifier-releases/issues/5
Quoted in that thread someone made a version with public source-code: GitHub - MichaelMeikle/ChannelWatchdog: Channel join notifier for TeamSpeak 3 client with persistence across sessions.
This could be updated to work for API 24. (Assuming it worked previously)
