Feature phones are still a thing, and they are even more private when you allow the battery to discharge completely. Track me now, guys!
I am not defeating the purpose, though. A charger in the car brings the phone to full function in a few seconds, in the event of car trouble or other unusual circumstance. It doesn't need to charge a bit first.
Shouldn't even have to discharge it with a lot of feature phones. Feature phones (at least the ones I've had) tend to have accessible, removable batteries. You could just open the battery housing and slip in a piece of paper over the battery contacts. When you need to use it, just open it and remove the paper.