I’ve owned and driven lots of cars both auto and manual trans. I think it is bad technique to use the left foot to brake. My main reason is that in a panic situation it is possible the driver would plant both feet on the floor and that means full gas and full brake. In most panic situations I’ve survived I needed full brakes, not full gas at the same time.
I have practiced left footed braking just in case I’ve got a bum right foot or ankle. I can do it but prefer right foot for gas and brake. I always teach new drivers to use only the right foot regardless of whether it is a manual or auto trans.