It was restricted to one very artificial game mode (1 vs 1 mid, Shadow Fiend mirror matchup) that is not representative of real games, but is good for practicing one aspect of Dota. Some items in this game mode are banned in order to make it less safe for both sides, and so that there is a better chance of one side dying before ten minutes. People have beaten the bot by both "breaking" it and by just outplaying it (but the latter very, very rarely).