GameCube controllersappear as "USB controllers", and GameXplain demonstrates that users are able to navigate the system menu and even play games like Fire Emblem Warriors with the old controller. Once you plug in the adapter into the console, open the settings menu and press the L & R buttons together. Nintendo has specifically enabled GameCube pads. The one feature that probably stands out the most is update 4.0 allowing Gamecube controller support for both the docked console and handheld version.
Being an old device, the GameCube controller doesn't have a Z button on the left-hand side which make it hard to use it some games. That being the case, we're guessing that Nintendo implemented this feature in secret in preparation for some kind of upcoming announcement that will make it relevant. That seems to be a bit optimistic. Unfortunately, the Switch has been severely lacking the usual Nintendo functionality during its 7-month life, which is surprisingly longer than it took to bring the feature to the now-defunct Wii U. Red Bull Games got to interview the developer and, interestingly, Golf Story was originally meant to be on the Wii U. Sidebar Games also revealed that Nintendo was very helpful in bringing their game to the Switch platform.
For the time being, it's just nice to know that Nintendo's strangest controller is going to survive yet another generation of gaming.