you can still buy 2K15 on Steam !

it seems you can still buy the game from official steam store : https://store.steampowered.com/sub/52292/

Looks it's regional, as it's listed at $0.00 and unavailable here in Australia. Same thing happened with NBA 2K10, 2K11, and 2K12, which is why I regret not grabbing the Steam versions when that method still worked here!