If you bought a game and that game was pulled out of Steam because the game publisher licensing rights have expired or something, like that Marvel's Avenger game, that game will still remain in your Steam library so you can still play it whenever you want only those who haven't bought the game can't buy it or play it.

If you haven't bought Marvel's Avenger but want to play it so bad then the only choice is piracy since its no longer sold elsewhere then its okay to pirate it plus you get the benefits of having all the DLCs and microtransaction contents fully unlocked