[2024] MOVED: When they shut down Alphas for good ?

There's no plan to shut them down so it is impossible to say. They like that people enjoy playing previous versions and there is no harm keeping them available. There was speculation by critics that TFP would close all the alpha branches when the game went 1.0 but that was proven untrue. Going forward everything I've read from the developers indicate that they plan to keep the old alpha versions of the game.