The Nintendo Switch's Joy-Con controllers will be supported in steam. Joy-cons will be supported either individually or as a matched pair, similar to how you can use them for Nintendo games. If you want to try the feature out, you can opt into the steam alpha, which has helpful instructions on the steam website.
The company didn't specify how it would improve support for Nintendo's classic controllers, but it did say that it would be better. The feature was added to the public version of the steam client last week.
You can also use a non-Nintendo controller with Steam. After the PS5 was released in 2020, Valve expanded their support for controllers from other manufacturers.