SDL2, the cross-platform development library has now merged in support for the Wii U/Switch USB GameCube controller adapter.
This work is the result of the successful IndieGoGo crowdfunding campaign from Ethan Lee, who previously ported a ton of games to Linux and Lee now also works with CodeWeavers to help with Steam Play/Proton development. This campaign was a personal project of Lee's, done across a few weekends.
From the commit:
hidapi: Add support for Wii U/Switch USB GameCube controller adapter.
Note that a single USB device is responsible for all 4 joysticks, so a large rewrite of the DeviceDriver functions was necessary to allow a single device to produce multiple joysticks.
This means that in future, you will be able to use the classic GameCube controller or perhaps even the DK Bongos if that's your thing. Considering the amount of projects (both games and game engines) that use SDL, eventually support for this will be quite broad.