Top of this list used to be particular versions of Ubuntu
Summary: GNU/Linux "market share" in Steam grew by a quarter this past year, boosted by SteamOS perhaps, but watch the trends in distributions; even the Ubuntu-based distros aren't near the top (and SteamOS moved from Debian to Arch, so expect Ubuntu to stagnate further)
Fact #1: over time slop gets worse (training set is like some blurry JPEG). Fact #2: People's "smell" for slop improves over time, as they 'train' on slop and can detect it based on prior encounters. Put 1 and 2 together.