e-bikes a separate category, since they add regenerative braking on top of everything else.

Actually, the vast majority of e-bikes do not have regenerative braking.

Because on a bike, you don’t actually tend to use your brakes very much or very often. And even when you do use the brakes, you’re slowing a smaller mass down from a lower speed (compared to cars with regen braking). There’s just not much energy there to be harvested from regen braking. Which makes it generally not worth the extra money, weight, and complexity to include a regen braking system.