It's amazing that secrets can remain hidden in some of the most popular video games of all time, but it does happen – and sometimes for several decades.
Take Zelda: Majora's Mask, for example; it has recently been discovered that the N64 version contains hidden support for the N64's microphone accessory, and that voice commands exist in the game's code.
YouTuber Skawo made the discovery, which could have been intended as a neat callback to the original Famicom Disk System version of the game, which allowed players to bark into the controller's mic. As the NES lacked microphones on its controllers, this feature didn't make it to the West.
While Nintendo decided against moving forward with voice commands in Majora’s Mask, the code still exists within the game. Planned uses included taking pictures with the Pictobox ("Say Cheese") and waking up Deku Scrubs.
The abandoned idea was only ever based on the Japanese version of the game, so you have to utter the commands in Japanese. Still, it's interesting to see that these cool secrets are still popping up years after release.