The latest Steam beta brought with it an unexpected change that may upset some sections of the user base. In short, the update added a new way for the client to verify a number of core details about any given game/app.
Steam does this by matching the app ID, depot ID, manifest ID, branch name and password with the most up-to-date app info. If a mismatch is found, the client can refuse the download. While this usually would be a good thing, by requiring this verification step, Valve is effectively preventing users from downloading anything but the most recent build of games/apps.
As SteamDB’s Pavel Djundik explained, this is specifically a problem for modders, speed runners and game preservationists, who often roll back to earlier versions for a variety of reasons, including reverting to more stable, pre-patch builds. It’s not yet clear whether these restrictions will apply to every game, or if they’re simply being added as a tool to be deployed by developers where necessary.