Recent changes to the SteamDB-entry of "The Stanley Parable" seem to announce an upcoming release for Linux.

about 2 hours ago Change #1177528
Changed App Config
launch/2/arguments: -game thestanleyparable
launch/2/config/oslist: linux
launch/2/executable: stanley

Changed changenumber – 1176958 › 1177528
about 6 hours ago Change #1176958
Changed oslist – windows,macos,linux
Changed changenumber – 912936 › 1176958