Totally agreed. The GM solution of using g-force is interesting. And the Ioniq solution of letter-of-the-law, showing nothing even during intense regen deceleration is clearly not safe.
While we're re-thinking brake lights, it would be nice to add some other affordances too. It would be nice to have an indicator of intensity of braking/deceleration, not just a binary feedback.
For example, why not add some strobing lights to extremely hard braking (like the sort of braking you might do if you're on the highway and there's a massive collision right in front of you). Something that unambiguously signals "extreme danger, prepare to react" to the people behind you.
Or a brake light color or shape that indicates you're at a complete stop.
Or more generally, perhaps brake light intensity should be a graduated, dimmer brake lights for gentle braking and intense ones for rapid braking.
Tech enables a lot more options and information these days, it'd be nice to explore making some improvements. EV transition is a good time considering some of our expectations are changing anyways.