The support is part of Steam’s beta client
Part of beta update: Steam continues to add support to Nintendo’s controller hardware. Notably, Valve (a video game company’s) continues to extend Steam to a range of controllers and as part of the newest beta update, the quirky Nintendo Joy-Cons officially get Steam support.
Valve spokesperson Kaci Aitchison Boyle confirms in detail that Switch-specific (games) NES, SNES, N64, Sega Genesis and Nintendo 64 controllers are actively compatible with Steam.
Play PC Games With Switch-Style Controls
Steam Client beta update, on the 4th of August, announced two changes, the most notable of which is:
Added support for Nintendo Joy-Con controllers, both individually as a mini-gamepad and combined into pairs”.Steam Beta – August 4
It may take a while for the beta update to go live on the main Steam client, so there is a little wait for it.
The Joy-Con support is available in the Steam beta update. To access this, you need to go into the Steam settings.
You can find “Beta participation” under the account menu. Click Change and select Steam Beta Update. Restart and you can enjoy the new feature.