https://gs.statcounter.com/os-market-share/desktop/worldwide
This graph shows the market share of desktop operating systems worldwide based on over 5 billion monthly page views.
Looks to me like desktop usage is on a downward trend in general, meaning it's probably more of an increase in casual Windows users leaving the desktop space entirely, relying purely on mobile systems.
Does Steam OS count as Linux, or Other; or is it not listed because it's not really a desktop OS, (while it could be used as one)?
It would count as linux. Though it probably only has a limited inclusion here unless they're getting stats from valve. Must of these get data from web traffic, which would only be when people are using a web browser.