I have no idea and I agree: The notification for forum replies was one of the few things that worked extremely well. Not to mention that the old notification was much more visible than the tiny purple dot we have now.
So, in typical GOG fashion scrap something useful and working well, and replace it with something meh that nobody asked for.
It doesn't tell you what replies you have, it's just another link to the "My posts" page that blinks every time someone replies to you.
GOG's really taking the piss.