Here's my two cents worth,
In the UK if you were
to use your left foot for anything other than the clutch during your driving test....instant fail.Even
if the test vehicle is an auto you are not permitted to use your left foot for brakes,in fact if you pass your driving test here with an auto trans you are not legally permitted to drive stick.
On the other hand if you pass in a manual trans car you are allowed to drive both.
The theory being it's entirely possible to overdrive the brakes with disasterous results as evidenced by the many Youtube videos of people barreling through shop fronts in parking lots.
Which I might add the majority of which seem to be from the USA,no doubt aggravated by your love of big torquey V8s,auto transmissions and/or perhaps left foot braking?
I've driven stick ever since I was a pup,in fact my current truck is the first auto trans i've ever owned in nearly 40 years behind the wheel.
However if I were to use my left foot to brake my muscle memory is now so pre-programmed to depress a clutch,you can just imagine the result so consequently my left leg is now just along for the ride...
My wife's first ever solo drive in an auto didn't go well,lets just say "thank goodness for seatbelts"!
She made the rookie mistake of using her left foot to brake.
That's despite my advice to basically ignore her left leg all together.