It might be a mistake with the gamecard. I bought Unrest: Special Edition some time ago and it features OS X and Linux installers, and I finished the game with the OS X version. I doubt that they had to remove those versions for the Special Edition, but you could ask someone who bought it recently to check, maybe someone in the Winter Sale thread.
I'm not interested in buying the game, nor would it matter for me as I'm a Windows user; I just found it enough of a glaring discrepancy to point out. Also, it's not just on the gamecard, but in the info shown in the search results for "unrest" as well.
I suppose I could have brought it to GOG's attention via Support, but it just seems weird to file a ticket for such an issue for a game which I not only don't own, but have no intention of getting. (Frankly, I'm surprised no on had previously mentioned this discrepancy in this subforum.)