So I got an email telling me that "Items on your wishlist are now discounted!" - but none of the mentioned games are on my wishlist. I guess they're games which I have removed from the wishlist some time ago.

So what I did now is unsubscribing from the wishlist notifications, and then subscribe again. Does anyone know whether this works as anticipated? Meaning I only want notifications for games which are *currently* on my wishlist.

I believe this is the thread you're looking for:

https://www.gog.com/forum/general/an_item_on_your_wishlist_is_now_discounted

Appears to need something on the backend reset. Try sending in a support ticket.

Hope this helps