In July Steam on Linux was at 2.89% but then for August it fell to 2.64% . Now for September it crept up to 2.68%.
Windows meanwhile was at 95.4% and macOS at 1.91%.
On a year-over-year basis Steam on Linux was at 1.87% , so basically up 1% year-over-year for September.
The Steam Linux numbers for September aren't too changed from August with SteamOS Holo coming in at 28% use among Linux gamers and Arch Linux at 10.69%. The AMD CPU marketshare among Linux gamers at 68%.
Those interested in all the Steam Survey numbers can find them on SteamPowered.com .