Just noticed while browsing game specific forums, that some of them list all the games that share the thread. Like Spellforce or Serious Sam for example (see screenshots).

However, other series like Doom seem incomplete and list only Doom 64. Is this so new that it's still work in progress?

I'm positive it's new. I just started seeing it yesterday.

I just noticed it today when i visited the settlers forum.

Hm. Neat.

I just noticed it myself - it has to be recent. I think it is a good idea to avoid confusion if any is possible with the forums. Personally what I have noticed sometimes I see people who don't often use GOG don't even know there are forums for every game/series so they post in the main forums which sometimes may help for smaller games but for bigger popular games I think it won't that much

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Seems to have been added to day. That's nice.

It must be new, as it wasn't there 3 days ago while I was reading the Legacy of Kain series subforum. I like it because it allows easy access to the game pages. However, I hope this feature remains consistent moving forward, considering that GOG often implements half delivered improvements and then quickly forgets about them. In such case, it's going to be more a problem than a solution.

Yeah, new, just reported in the changes thread too.
But about that issue with series forums existing but not including all games from the series, I'd say it's a GOG problem. The proper way would be nesting, series forum with subforum for each game, but since I guess the forum software won't handle that, I'd always go for separate forum for each game, yet if that's not wanted at least in cases where the separate forum is just for the most recent game, or at least the more recent games, they could rename the "series" forum to something like "classic" or "legacy" "series" to at least give a hint.
Not that there's any consistency. Witcher games get their own separate forums, Baldur's Gate has a series forum, a BG3 forum and a DA forum, Divinity gets a series forum and one for D:OS2, AoW just has a series forum, Spellforce gets a series forum and one for CoE, TES just gets a series forum, let's say that Fallout getting a series forum and one just for London makes sense, VtM just gets a series forum for both the (now) classic RPGS and the new VNs which really makes no sense, and so on.

Also Trails of Cold Steel has a general subforum and a ToCS4 subforum, and you can end up in either depending on which ToCS4 link you follow.

It's not funny, how they spend time on things like this that aren't really needed, while other things that actually need doing get ignored.

At best you could say, it gives a more professional look ... maybe.

Better still, we all stumbled into this feature instead of it being (even quietly) announced anywhere on the site. Is it announced on their blog?

Checks.

No, of course not. That was last updated in June with news about Luna. Imagine GOG as a beekeeper.

"Sorry you got stung, I was trying to tame Killer bees and forgot to mention I even had hives installed in the first place."

My thing has been long for quite some time (assuming Hummer didn't already make this joke).