I remembered that I saw it on the Volt forum back when I was more active there so that greatly narrowed down my search. See post #4 in this archive:
https://gm-volt.com/forum/archive/index.php/t-181089.html
Spoiler: the technical terms are fairly meaningless to the average joe like me.
So this is what I was looking for, it seems they have set a standard, although it only applies to electric vehicle regen:
"Electric regenerative braking systems as defined in paragraph 2.17., which produce a retarding force upon release of the accelerator control, shall generate the signal mentioned above according to the following provisions:
≤ 0.7 m/s² The signal shall not be generated
> 0.7 m/s² and ≤ 1.3 m/s² The signal may be generated
> 1.3 m/s² The signal shall be generated"
This refers to meters per second per second. In the U.S. we would relate better to MPH per second, or even better MPH per ten seconds, which would look like this (rounded):
≤ 15 MPH deceleration per 10 seconds - The signal must not be generated
> 15 MPH deceleration per 10 seconds - The signal may be generated
> 30 MPH deceleration per 10 seconds - The signal must be generated
So ICE cars are ignored, thus you can lightly tap the brakes and activate the brake lights even though you are barely slowing down. Manual transmission is ignored, so you can downshift and do heavy deceleration without activating the brake lights.
I think the same standard should apply to all vehicles. Instead similar to pedestrian warning sound they only seem to make regulations for EV, even though ICE cars can also be nearly silent when coasting at slow speeds. It's time to get the brake lights standardized, after all a driver has no way to know if the car ahead of them is EV/PHEV or an ICE with automatic transmission or an ICE with manual transmission.
In my proposed method brake light activation for all vehicles would be mandatory above 0.7 m/s² and flashing brake lights above 1.3 m/s². Of course there will be several years of overlap since it is not feasible to retrofit previously build vehicles except as optional aftermarket, but that was true with third brake light also. But the sooner they standardize then within about ten years most cars will work the same. Of course maybe all new cars will be electric by then, who knows, but either way if they delay this then it will just be that much longer with brake lights becoming increasing indecipherable, or I should say even more so than they already are.