The Nintendo Switch now supports GameCube controllers !! By connecting a GameCube controller via the Wii U USB adapter, the Switch console will now read it, allowing players to use the controller with seemingly any Switch game. Once hooked up, it’ll be detected as a USB pad in the Switch’s controllers menu. It doesn’t look like other USB controllers are supported at this point, however.
Update : It appears that this was a "happy accident", rather than a clue to anything more intended update. However, speaking to Kotaku, Nintendo of America president Reggie Fils-Aime explained that: “In our latest system software update, it enables third-party peripherals to work on Nintendo Switch. As a result, the peripheral for Wii U that allows it to attach GameCube controllers also works [...] I think it’s fair to say [...] that that peripheral working was as much a surprise to us as it was to consumers.” Sourcs: IGN