Valve has reportedly been removing SteamOS icons from Linux titles on Steam, with the reason behind it being that these games don't meet the requirements of SteamOS, or they're experiencing compatibility issues.
Gaming on Linux reports that some of these games are no longer available on the Steam store, but if you own the game you can still download and play them. StarMade, one of the games in question, was removed from Steam due to a Java requirement. The developer has said that once the new launched is available, the issue will be resolved.
Valve most likely doesn't want to advertise a game as having SteamOS compatibility, if an "outside launcher/installer such as Adobe Air, Flash or the like", according to Gaming on Linux.