Do Steam keys still activate the Mac version? All the linked storefronts have removed Mac from their listing.
Topic on Talk:Deus Ex: Human Revolution - Director's Cut
Probably.
Steam used to have "Steam Play" games which was games where you bought the game once on the platform (or unlocked it using a Steam key) and got it on all supported platforms, meaning Windows, OS X, and Linux. Those games used to be defined separately on the storefront as not all games were Steam Play games.
Nowadays, however, to my knowledge all games on Steam are of this sort -- if you use a Steam key or purchase the Windows version of a game on Steam, you will automatically receive access to any OS X or Linux version that might exist on the platform.