Last year, it was discovered in the Switch's 4.0.0 update that the GameCube adapter was compatible. This meant that the popular GameCube controller was usable on the Switch. This, however, seemed unintentional, as even Nintendo was surprised. Reggie Fils-Aimé told Kotaku back in October that "I think it’s fair to say, Stephen, that that peripheral working was as much a surprise to us as it was to consumers." While not unofficial, the GameCube adapter working on Switch doesn't seem like it was supposed to be a feature. The reason for this could be that Nintendo may be releasing GameCube-styled controllers specifically made for Switch.
Over at Reddit, a user named Mew_The_Creator posted an image he claims to be "Official Licensed* controller mockups." According to the user, the packaging was sent to the user's facility, an American Walmart not opened to the public (probably some kind of warehouse), and that "The controllers are confirmed and have a product number to be sold eventually." Analyzing the picture yields some interesting observations.
If you look at the bottom right on the packaging, you'll notice the "PowerA" logo. PowerA makes officially licensed Nintendo controllers. If this controller is actually coming out, then this is a product officially licensed by Nintendo. As for the actual design, one can see that it is clearly based on the GameCube style. However, it does feature key differences. Instead of having one "start" button, it has the +, -, and home buttons. This makes sense, because simply porting over the GameCube controller to the Switch was not going to work. If you tried using the GameCube adapter, you probably found that while it works, it can't do everything the standard Switch controller could. This supposed new controller solves that problem. However, it is wired, meaning that the gyroscope function will probably be absent. Mew_The_Creator also posted an image for what looks to be upcoming PowerA Mario-themed wired and wireless controllers for Switch.
It wouldn't be a stretch to say that the GameCube-inspired controllers may be on the way because of the upcoming Super Smash Bros. Since Melee, the GameCube controller has become the standard for the fighting game series. Nintendo is aware of this, because the company released a GameCube adapter just for Super Smash Bros. for Wii U back in 2014. Releasing GameCube-styled controllers for Switch is a sound move. Perhaps Nintendo will officially announce it next week at E3. It would be a perfect, low-key but highly applauded announcement. Of course, nothing said here is official or confirmed. If these wired controllers are accurate, we'll probably hear about them soon because Nintendo would surely want to release them in conjunction with Super Smash Bros. for Switch.