No idea, I never noticed until I saw someone mention it. There wasn't a hands on with the SP4 and the SPB video was very short. Maybe they changed it to a gesture swipe? What happened to the Infinity-like display tech that was rumoured? Maybe it lights up on the bezel whenever you change orientation?
The start button will most likely be your Windows 10 Start Menu button. In Windows 10, there is no need for a dedicated Windows key on the frame since, all it does, is to open the start menu.