I dunno if you'd count this one on a technicality, but Freespace 2 on Steam comes to mind. It's not as much the game is "inaccessible", but it relies on fanmade tools and programs to be made functional. If you go to the store page and look at the reviews, this is one of the first things you see:
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Indeed, to play this game you paid money for on Steam, you need to download third-party software. The game may get a pass solely for its age, but there's no excuse to sell a broken product.
I would also extend this to pretty much every game that was released on Gaming For Windows Live. Not only are the servers no longer available, but the programs themselves don't play nice with modern operating systems. Something like MAG on the PS3 comes to mind as well.