Valve is also part of the Accessible Gaming Initiative, along with Microsoft and others.

And Steam does already support the Xbox Adaptive Controller, along with a huge amount of control overlays, and if that isn't enough - SteamInput can be disabled for raw HID mode and then you can interface with any 3rd party app, like in the article.

Their virtual menus are a godsend for building your own control systems - I've used one with eyetracking software to play Counter Strike.