In Unity, How to Stop Popup for "Show Unity Splashscreen"

The location changed. I found the bool recommended_ShowUnitySplashScreen in Assets\SteamVR\Editor\SteamVR_UnitySettingsWindow.cs.

Go to Assets/SteamVR/Editor/SteamVR_Settings.cs and change recommended_ShowUnitySplashScreen to true. You'll have to do this each time you update the SteamVR Unity Asset, until they fix it to check which edition you're using.