Joy-Con support for the Nintendo Switch is being added to Steam

As part of the most recent beta update, Valve included support for Nintendo Switch Joy-Cons, thus broadening the selection of controller

The August Steam Client Beta community update article features a fairly brief list of updates, with Nintendo Switch Joy-Con compatibility

Not only does the article state that Joy-Con support for the Nintendo Switch has been enabled, it also adds that Joy-Con

Only the Steam Client Beta has support at the time. In the “Account” area of the Steam settings menu