I was thinking about this last time I drove an ev (ioniq 5). It will really decelerate quite hard when you lift off, and it’s configurable by the driver.
I don’t think they need to do it with an accelerometer, but if the regeneration system is applying more braking force than it would take to turn on the light with the brake pedal, it should turn the light on.
Either that or they should require the brake pedal to be used beyond that point.
Edit: actually it just occurred to me that it might be no worse than downshifting in a normal car. Maybe it’s not a big deal.