A few people have mentioned similar things and it’s a bit of a different situation. For all Steam knows you got that code in a Humble Bundle and are just now getting around to activating it. A bit more of an extreme example would be Prey (2006) which was removed from Steam I think in 2009 (oddly enough because they ran out of keys) but yet you can still find and buy keys for the game.
What I am talking about would be more like Afro Samurai 2, Alan Awake, or Star Control where games were explicitly removed due to some kind of special circumstance and not replaced. Would there be a point where Steam simply has to remove a game from a users library?