I have definitely seen the dealer enforce the “show one, show all” rule. I tried googling around after your last comment, but wasn’t able to find any reference to a player’s right to see a mucked hand. Instead, I found a lot of articles about whether it is better play to show or muck losing hands, which implies it is the person’s choice.
Do you have any reference for the rule that a folded hand has to be shown on demand if play is over?