Not required by law, but will probably be implemented in the same fashion as the Volt.
If regen paddles are used, they will come on. They will also come on in other circumstances depending on drive mode, speed, throttle position, etc.
Thanks. I figured you'd want the car behind you to know you were slowing faster than being in neutral, but I'd not seen anything on this. I haven't had a vehicle with regen.
Mild regen doesn't make the brake lights come on - this is just like any ICE, which slow down when you lift your foot off the accelerator. It depends on how quickly you are slowing down.