Valve has recently confirmed that Steam will now support Nintendo Switch Joy-Con controllers.
To clarify, the Nintendo Switch Joy-Con controllers are not compatible yet with the public client but are currently on the latest client beta. Steam players who have joined the beta test can now enjoy playing games with the use of the Joy-Con controllers in both individual and paired configurations.
Players can play games with individual Joy-Con controllers, which could function as mini gamepads. Combine the two controllers to have the normal configuration of a regular controller supported by Steam like the Xbox, DualSense, and DualShock controllers. Do note that the public Steam client also supports the Nintendo Switch Pro Controllers.
With this, Steam has now come full circle with Nintendo as it supports all Switch official controllers. These are the Joy-Con and the Pro controllers.
Aside from the modern controllers from Nintendo, Steam also supports the classic ones like the NES, SNES, N64, and Mega Drive, which was introduced last month. Now, the beta client improves the support for those. It will be available on the public Steam client possibly in the near future.