I voted but would like to see something like this.
Tap on an aircraft not on your frequency without sending him a message and the tag toggles on. Tap it again and it turns off. I find it’s a pain to have to keep tapping on someone that’s approaching your airport at FL200 to see if he’s descended into your airspace. This handles that case, and also handles your case! Tap on someone you’ve given freq change to, and his tag turns off. Tap it again and it turns on.
The difficulty I see with always turning it off on frequency change is with tower when you give freq change, but he stays at Pattern altitude. He could be coming back in, but with the tag off, you’ll miss the fact that he’s not climbing.