I saw some of those on some US army vehicles on the German autobahn. And what perfectly illustrated their danger was when they almost got rear-ended while entering the left-lane in front of a passing Audi at mild autobahn speed.
Had it been a yellow blinking light, the left-lane driver would have been better alerted to the fact that someone was about to go left. Instead, it was a muted blinking red, at the same intensity as the car's tired red back lights, that looked like nothing more than a defective back light.
Such a stupidly dumb design.
such as indicators in the same color as the rear lights, are perfectly legal.
In any European car you get two lights, not in the center but in the corners so you can actually see stuff in your side mirrors while parking.
A bunch of Japanese compacts and subcompacts do it too and it was basically unheard of on any vehicle from any continent until the last 15yr or so when backup cameras proliferated.
Confirmation bias is a hell of a drug.