When I bought my last car two years ago, one of the cars I looked at was a GM car. They tried to upsell me on a feature that would flash the brake lights intermittently when you took your foot off the gas, thus warning those behind you that you might brake. This was a monthly subscription. In the past two years, I only seen this on the road 2 or 3 times.
Oh I see it fairly regularly now. Have a couple friends that have it that said the dealerships almost didn't make it sound like an option. I think it's unnecessary and a waste of money, but I do notice when cars have it.