I worry this could add to "Road Rage" as people think your brake checking them (and technically, you would be).
As the driver, I don't want a warning that I'm being tailgated. Unless my plan is to again "brake check" them.
Let me worry about driving. I have a review mirror if I need to know what is going on behind me, I don't need an automated system to tell me there is a jerk behind me as my normal situation awareness should include checking my mirrors as part of my scan of what is going on.
... as another segue... the algorithm for activating the brake lights during either driver induced regenerative braking or Adaptive CC should adjust for the distance, speed and approach speed differential of the vehicle behind. This makes so much sense from a safety POV I can't imagine it not being implemented.
I will copy this suggestion to a new desired feature just in case it hasn't already.