In a secret addition to the 4.0.0 update for the Nintendo Switch, the GameCube Adapter released alongside Super Smash Bros. for Wii U is now compatible with the Nintendo Switch when it is in docked mode. Found by Master Mewking on Twitter, the GameCube Adapter allows GameCube controllers to be recognized as a USB controller by the Switch system.
— Master Mewking (@MasterMewking) October 23, 2017
Any games that will support USB controllers should work with GameCube controllers including The Legend of Zelda: Breath of the Wild and the recently released Fire Emblem Warriors.
What’s peculiar is that the GameCube Adapter was stealthily added in this update and was not available prior. There is a possibility that Nintendo could be preparing for a Virtual Console service or even a Super Smash Bros. entry. USB controllers not developed for Nintendo systems, such as a wired PlayStation 3 or a wired Xbox 360 controller, will not work with the system.