I think it's probably a temperature dial and I have no idea if cigarette lighters are still in cars these days. Not really something I pay attention to.
That's an odd place to out an outlet so we can probably scratch that off the list.
The actual element that gets heated up to light a cigarette has been disappearing from new cars but the same 12v plug is now used as a power outlet. So for as long as you can imagine that will live on. But at the same time we're seeing car makers include dedicated power ports... USB charging ports being the most popular right now.