I have tested the regen and the brake lights change in different circumstances. During daylight, the brake lights do not come on for any level of regen. At night, the brake lights come on at 3 chevrons over 40 mph and at 4 chevrons under 40 mph.
This is weird--I did some brief testing tonight and determined that in the dark the brake lights didn't come on for regen
at all, even with four chevrons, at any speed 35mph and under. I wasn't able to test at higher speeds.
I say weird, because that not only doesn't line up with your testing, but it
also doesn't line up with daylight testing I did a few months ago with someone behind me on the phone. At that time, the brake lights
did come on with four chevrons of regen at speeds well below 35mph. I did just have a bunch of software updates done by the dealership, but I don't remember brake lighting being mentioned in any of the bulletins, and that seems like a surprising thing to change. It's harder to test in daylight, but now I'm going to go try again and see if I get different results than last time.
So at least in my car, at night, the brake lights no longer come on at any time for <35mph regen braking until I hit the pedal. Which is actually kind of unsettling.
As for downhill ACC, the brake lights come on for any deceleration beyond about ¼ to ⅓ of the green band (that is, halfway or two thirds of the way between 0 regen and the white tick at the midpoint of the green band). That is true even with cruise control set to 25mph. In that case, there is a very audible click from somewhere under the steering column when the lights come on--I had heard that previously and suspected it was the brake lights activating, but tonight I confirmed that definitively.
This means that the brake lights come on pretty regularly if the ACC is following another car on the highway that's going a bit slower than the setpoint or changing speeds--I hear that click quite often. So ironically, under relatively normal conditions the brake lights are coming on all the time making it look like you're impatient and hitting the brakes repeatedly while tailgating someone, but even when coming almost to a complete stop with regen only they don't, making it look like you're not braking at all.
Technically the regen behavior isn't any different than using engine brake in a manual transmission and the ACC isn't different than being one of those people who repeatedly hits the brakes at highway speeds when following someone, but neither is the beha