The reason is that old toaster designs had unpolarized plugs and therefore a good chance of uninsulated live wires inside, even when off. Modern toasters don't

The reason is that old toaster designs had unpolarized plugs and therefore a good chance of uninsulated live wires inside, even when off. Modern toasters don't anymore, but folk wisdom lasts forever. Although you make a good point that if your outlet polarity is reversed, you might still have a problem. I don't know if modern designs are robust to that... I learned this from a video about old Sunbeam toasters that was unexpectedly fascinating: https://youtu.be/1OfxlSG6q5Y https://youtu.be/1OfxlSG6q5Y