Valve has removed the SteamOS icon from a batch of games compatible with Linux on Steam, meaning that Linux owners now cannot download those titles anymore.
Reportedly the reason behind the removal of the SteamOS icon from these games is because Valve had determined those games do not work properly with their new OS, and so ahead of the big launch of their new Steam Machine line have trimmed those games to avoid problems later on with games not working which could cause more work around a time they will be busy with the hardware launch.
Unfortunately because the Linux Tux was replaced earlier this year with the SteamOS icon, it does mean that regardless of whether or not a user is using SteamOS they cannot purchase and play these games any longer. However, games purchased before this removal can still apparently be downloaded and played as normal.
Steam Machines, Steam Link and Steam Controller all launch next month.