Nintendo has added support for the GameCube controller to the Nintendo Switch in version 4.00

The recent 4.00 update for the Nintendo Switch comes with a secret: It brings GameCube controller support to the Nintendo Switch.
By connecting a GameCube controller via the Wii U adapter, the Switch console will now read it, allowing players to use the controller with seemingly any Switch game.
When you hear the GameCube controller being talked about today usually it has to do something with Super Smash Bros. A lot of pro players use the controller to play competitively and it is arguably the best controller Nintendo has made to date.

We could be getting closer to a Smash Bros. announcement in a direct in the coming weeks to months or in early 2018.
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Stay tuned for more updates on this.