My understanding is that for a long time now modern sockets don't use the holes for detent and instead just rely on friction. The current ANSI/NEMA standard says that: "The hole in flat blade is optional, and it is intended for manufacturing purposes only. However if used it must be located as per dimensions shown above"

Some interesting discussion on this topic here: https://electronics.stackexchange.com/questions/11495/nema-5...