Some people may have never seen one (or even a photo of one) but the earliest IBM PC models had a big red switch on the side, built in to the PSU. It was an up/down flip switch, made of heavy plastic. It required some deliberate effort to move. It was at the rear of the case, with a black plastic surround. The metal case had a square cutout to fit it.
Powering off was done fully in hardware.
Software packages usually advised users to exit cleanly first.