Where is the ruling that hidden cards need to be revealed at the end of a game even if they weren't used?

According to Rule 421.4 of the Core Rules, you must reveal all facedown cards at the end of a game:

"421.4. If a facedown card would change zones or if the game ends, its owner reveals it to all players. See rule 128. Privacy for more information."

This ensures that all players can verify the cards that were played throughout the game, regardless of whether they were activated or remained hidden until the conclusion of the match.