If you're a stickler for the older hardware, preferring the feel of the GameCube's asymmetrical controls to these modern 'Pro' gamepads, you're in luck. The Switch's 4.0 update (which dropped last week) came with support for the classic GameCube control pads.
Because Nintendo doesn't really do patch notes, people have spent the last several days working out exactly what changed when last week's update hit. USB headphones are one notable improvement, as is the option to grab in-game footage and the ability to transfer save files between consoles. But as the below video from GameXplain shows, GameCube controller support came along for the ride.
You will need a GameCube controller (obviously) plus the USB adaptor originally released for the Wii U. Plug them both into the Switch's dock and it'll register the controller as a USB gamepad. Nice and easy. Presumably this won't work in handheld mode, because there's no wireless connectivity between the Switch and its dock.
So what does this mean? Who knows. The GameCube controller is a long-time favourite of serious Smash Bros players, which might hint that a new instalment of the fighting game series is on its way. It could also simply be that Nintendo knows people like the GameCube controller, and just wants to give people more choice. Plus if people like using it for Smash Bros. maybe they'll start using it for games like Pokken Tournament.