Older Game Versions No Longer Playable Through Steam

Pavel Djundik tweets about a change in how the latest Steam beta handles game updates that no longer allows users to play previous versions of games (thanks PCGamesN). The data spy who founded SteamDB explains why you can no longer downgrade games via Valve's online service:
Last Steam beta introduced an important change to game downloads.

With this change, it is no longer possible to download older game versions, because client asks for a code (GetManifestRequestCode) that rotates every five minutes.