Putting 0 A.D. on Steam has come up a couple of times, for example here, and here. Unless the team's position has changed, the perspective is that bringing 0 A.D. to Steam would require a more mature version of the game. The current work-in-progress state of the game might be detrimental to the public's opinion, which could hurt the long term prospects. I don't think it's ruled out for a (near) final version of the game.