When discarding the card in the Love Letter, is it mandatory to show the card you had?
The Tempest version of the rules is just a little bit vague - it does say that to play a card you discard it face up, it does say that your discarded cards go in front of you, it does say that when you're knocked out you place your card face up in front of you, it doesn't explicitly say that a card you discard to the Prince should be face up (although I'd say that they weight of all the other rules says yes it should).

The Kanai version definitely spells it out: 


All discarded cards should remain in front of the player who played them, visible, so that it is clear in what order they were discarded.
If a player is out of the round he or she must reveal the card in his or her hand (if any) and place it in front of him or her.