In modern toasters in modern homes. The trope absolutely did stem from this being possible in the past at least. Many toasters would not have a fuse, and older homes and electrical wiring did not have GFCI outlets in bathrooms and kitchens. So you could be electrocuted until you tripped the home circuit breaker. And jamming that into the closed position would be all you have to do before your electro-dunk in that case.