SteamVR SDK Support is currently provided through the use of an integration package. If you require Steam device support (such as Valve Index or HTC Cosmos) you may use the OpenVR plugin along with XR Plugin Management, the SteamVR SDK, or the (preview) OpenXR Plugin.
If you want to give SteamVR a go, this integration package can get you started. It includes SteamVR bindings you can use to map SteamVR actions to raw inputs that the InputBridge can read.
DeviceSupport Installation instructions found here.
Note : This essentially maps SteamVR Actions such as "Grip", "Trigger", etc. so that the InputBridge can convert it to be used as raw input. It's not how SteamVR's input system is intended to be used, but is currently the only way to get input from certain devices until Unity gets full OpenVR or OpenXR support.
Check out InputBridge.cs to see how Steam Actions are bound to inputs.
Below is a video that walks through the process of integrating SteamVR with VRIF :