Nintendo really needs to get better at writing changelogs. After the unexpected discovery that Switch firmware 4.0.0 brought USB headphone support, the above GameXplain video shows how the console can also now be used with GameCube controllers.
You’ll need the GameCube controller USB adapter that Nintendo released for use with Super Smash Bros. for Wii U. 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.
Unlike the Wii U, the Switch supports GameCube controller input for just about any regular game as long as you can make do without a ZL-equivalent shoulder button and the home button. You'd have to play through Breath of the Wild without using your shield, for example.
It’s good news for prospective players of the inevitable Smash Bros. for Switch, as well as anyone hoping that Nintendo might offer GameCube titles for download someday — games like Super Mario Sunshine would be tough to play properly without the GameCube controller’s analog triggers.