Is it actually the case that Unrest and the Unrest Special Edition Upgrade are available for Windows, Mac and Linux, yet the proper ready-made Unrest Special Edition is only available for Windows, according to the respective store pages (and the search listings)? If this is true, I have to ask GOG: WHY??? How does that even work, let alone make sense?

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.)

It appears to be fixed now, for me at least. In cases like this one instead of writing to support, you can just reply to or PM Judas over on the 'What did just update thread'. I think he's got enough privileges to change the game cards like that. For more serious issues, it's definitely important to contact support.
http://www.gog.com/forum/general/the_what_did_just_update_thread/

Thanks for the heads-up. =)

No problem! :)