Comment by ThatPlayer

SInput recently released and got supported by SDL, which plenty of games, but also Steam Input uses. So you can already use SInput in Steam Input. Better than XInput for sure.

https://docs.handheldlegend.com/s/sinput/doc/sinput-hid-prot...

I don't think Steam has ever published specs for their protocol. And without Steam, their old controller would fallback to a mouse/keyboard mode. The Linux kernel drivers (that didn't require Steam) were reverse engineered. Hori released a Steam Controller recently. Even that still had an XInput fallback switch.