I'm not saying I'm worried about getting singed, but it left a bit of a foul taste in the mouth when the gog newsletter had an extra discount for Sinking City just the day after it came back.

?
-50% is not good enough?

We asked same question on Discord server, GOG staff responded that there aren't any additional discounts planned

I'm not sure you actually read the context I wrote around the question. It's nothing to do with the size of the discount, it's the issue of whether or not they're going to immediately offer a discount on top of that.

For example, let's say they were selling it for 10 bucks, but discounted it to 5, and so you buy it. Then, immediately the next day, they offer it for 4.50, you're going to have a reaction to that. I will not press you for how you'd react, because either you'd get it and I need say no more, you wouldn't get it and nothing else I say would explain it, or you would get it but would refuse to acknowledge it in which case you're not an honest actor.

Also, as I said, they did this just weeks ago with Sinking City's rerelease, so there is cause for concern (which imlinguin says has been assuaged, making this moot).

From my experience these come normally hand in hand with the sale on the main page or completly separate from sales.
If you overlooked it or they made a mistake and did send it too late, i am sure you can ask out the support, they are normally nice in such regards.

That's been my experience as well. And yeah, GOG's cs is pretty good, but if I can save everyone trouble by waiting a couple of days, that just seems the easiest route.