Since engine braking is an extremely common way for motorcycles to reduce speed, there are any number of aftermarket products that are essentially an accelerometer inline with the brake light, which will turn on or flash the brake light when sudden deceleration is detected.
I can see no reason why this wouldn’t work equally well for cars.
I was curious on this, so quickly checked a search. Some quora answers indicated it is a matter of regulation. General feel I got was it was expected to change with newer vehicles, but inertia in the current way things are certified will be a barrier.
I can see no reason why this wouldn’t work equally well for cars.