Current problem
There's no way to receive Notifications when somebody joins a Voice channel short of making a custom bot.
Proposed change
In the notifications menu of a (voice)channel (where you toggle push notifications etc) let me enable Notifications for join events so I get a notification when somebody joins.
Potentially more problematic (and should be able to be disabled by the person in question): A setting to receive a notification when a (select) friend joins any VoiceChannel that I can also see
Platform
macOS, Windows, Linux, Web, Android, iOS
Additional information
Discord has this feature called 'Voice Activity Notification' (on Mobile) where they decide for you (ew) which of the people you regularly are in VC with you will receive PushNotifications about.