As long as you have a Steam account, you can redeem a key through the client or the web interface. It's fairly straightforward, but Steam's knowledgebase has an
article with the specifics.
Generally speaking, if you redeem a key for a game that's no longer listed in the Steam Store, it'll still work and you'll be able to download the game. Getting hold of such a key is another story, of course. Sometimes you can find one on eBay or Amazon, but if not, you've more or less got to take your chances with a grey market key reseller like G2A. As long as you have a valid key though, you'll still be able to get your hands on NBA 2K16 PC.
I don't have any personal experience with those resellers, so I can't make any recommendations as to their service and trustworthiness. Someone else might be able to offer a recommendation, but you'll see some options and prices from the various resellers if you Google "NBA 2K16 Steam Key". I know others around these parts have mentioned using G2A before, but I don't have any experience with them myself. Just make sure that you get a key that'll activate in your region, or a global key, otherwise you'll be throwing money away on a key you can't use.