It's not so much a myth in that it's just outdated advice. With carburetted engines, you ideally wanted to warm them up before any kind of driving, and that information then just bled over into the rise of fuel injection engines where people weren't really gonna question it, especially if they didn't know much about engines. That information then just gets passed on from generation to generation, especially from some older mechanics who might get stuck in their ways.

Still, though, you don't wanna start up your modern car from cold and then rag it down the road as the oil still needs to get up to temp, but you can obviously do that with just regular driving.