Steam is ever-improving and constantly expanding what is supported by Steam Input. Just recently they added a new Steam Client Beta with support for the Nintendo Joy-Con controllers.
This follows on from the recent stable Steam Client release which added support for the Nintendo Online classic controllers including the NES, SNES, Nintendo 64 and Sega Mega Drive / Genesis controllers.
With the new Beta released on August 4th Valve has now added the official Nintendo Joy-Con controller support, that can be used individually as a mini-gamepad or them both combined. Additionally, support for the Nintendo Online classic controllers has also now been improved.
So a tiny update in terms of what's new but still a very welcome one.
It's actually pretty incredible just how wide the controller support is on Steam now. It's likely you can just pick up close to any controller and it will work.