Source: Nintendo
Steam now supports the Nintendo classic controllers for Switch Online. An update released on Wednesday noted that you could use your NES, SNES, Nintendo 64, and Sega Genesis controllers made for Nintendo's Switch Online service with games on Steam.
The NES and SNES controllers are available now, while the N64 and Genesis controllers are currently out of stock. These controllers are only available to Switch Online subscribers. Switch Online comes with a catalogue of Nintendo and Sega retro games.