After surreptitiously adding wireless headphone support with the latest 4.0.0 update, it's now confirmed that Nintendo also added Switch support for GameCube controllers. Hopefully, Switch owners haven't already thrown out their old GameCube controllers.
It was first discovered by a Twitter user named Master Mewking and then tipped to GameXplain by another Twitter user, Dark Throwback. GameXplain uploaded a video to their YouTube channel showing off the feature and how to set it up.
Unfortunately, it's not as straightforward as it seems as you'll need to have the GameCube USB adapter for the Wii U. However, once it's plugged into the Switch, it's simply a matter of going into the controller menu on the Switch and pressing the 'L' and 'R' buttons together. GameXplain goes on to demo the GameCube controller by playing Fire Emblem Warriors. It must be noted that other USB controllers like the Xbox 360 controller do not work.
By nature of it having a different layout, the GameCube controller does take some getting used to. For example, the GameXplain video notes that the GameCube controller lacks a left 'Z' button and only has the Start button so there's no way to go back to the main menu. Also, some of the button combinations the commentator was used to on the Switch controller changed slightly when using the GameCube controller.
The larger takeaway from this discovery is that this could hint at Nintendo adding a GameCube virtual console to the Switch. Super Smash Bros. players could possibly use the GameCube controller for any future Switch version of the game. In the meantime, if you're looking to relive the glory days of the GameCube (at least the controller), this is a surprising yet welcome addition.