When Super Smash Bros. for Wii U released, it was almost impossible to find an official GameCube controller adapter, so many fans opted with the third-party Mayflash adapter. It’s recently been revealed on Reddit that the device now functions on Switch like a Pokkén controller.
The above video demonstrates the Mayflash GameCube Adapter working on Switch with a new firmware update that supports Pokkén Tournament DX. The device appears to have trouble with various ports and isn’t fully compatible with all games as issues with The Legend of Zelda: Breath of the Wild has been noted, but it works perfectly fine on Puyo Puyo Tetris.
Nintendo’s own GameCube Adapter for Wii U still doesn’t work on Nintendo Switch, but if it doesn’t by the time Pokkén Tournament DX comes out, then the Mayflash will likely be your best bet if you want to play using a GameCube controller.
However, do keep in mind that the Nintendo Switch’s recent 3.0 firmware update made the Wii U Pokkén Pad compatible, so there’s always that option as well.