The Valve Index is now a distant second with 17.3%, overtaking the discontinued Oculus Rift S on 16.25%. HTC's Vive now fails to break into double-digits at 9.31%. Windows Mixed Reality combined sales equate to just 5.55%.
The survey pages states, "Steam conducts a monthly survey to collect data about what kinds of computer hardware and software our customers are using. Participation in the survey is optional, and anonymous. The information gathered is incredibly helpful to us as we make decisions about what kinds of technology investments to make and products to offer."
It's worth bearing in mind that as part of an 'optional' survey of users on one storefront, the data isn't definitive, but it's the best yardstick available at the present time for us to gauge market share.