My "sources" are posts in some similar threads here on this board.
I'm sorry I cannot be more specific than that.
But if you check out the person who created the above petition, I'm sure he posted in one of those threads.

Good news everyone, GOG replied to my request on the same day (10 October) and provided me with ftp details for old gothic 2 installer (with option to not install Night of the Raven).

Meaning that we still have that option.

Thank you very much for the notification.

Makes one really wonder why they did not make this option available for everyone by default. Because the description on the game's page clearly states that buying the Gold version will enable you to play Gothic 2 and the expansion - which no longer holds true.

Instead, they are forcing those interested in the vanilla game (which, in my view, is way more enjoyable than NotR) through a labyrinthine quest on these forums just in order to get what they already paid for ...