A lot of US cars use the same physical light for turn signals and brake signals, but I'm saying noticing the car in front of you is slowing down because the brake lights are on can be a more reliable way to know someone is about to turn because no one uses turn signals properly

Not universally. And even for cars that do, it is still easily noticeable when someone has their signal on. There are at least 3 brake lights, so if one of them is flashing instead of solid, that one is the turn signal.

I find more cars these days tend to have dedicated front and rear turn signals that are separate from the brake lights, though.

It still flashes. It's dumb that we don't charge it, but it's still pretty noticable in all but the quickest stops.