So, some thoughts about folks' responses:
I think FGD was mostly always about giving a space for developers to interact with players/playtesters.
In theory, this is actually a very valuable service. One of the biggest issues I've had over the past decade or so of libre gamedev is actually reaching players, so I think a site built around this notion is actually a pretty good idea. It makes me wonder if the issue is more one of image, since that goal wasn't really clear to me when I joined.
I do agree that a lot devs have built their own platforms now, though smaller/solo devs can still have a lot more trouble since they need to balance that community building with their own dev work.
And we have less people but have to do the same job. And also important, we have less capable people I guess. Sure we have coding and art geniusses. But less than the industry has.
I don't think the industry is a good target, personally--The vast majority of public-facing gamedev and dev discussions are by and between indie devs rather than salaried folks working at bigger companies (Mostly because of social media rules and NDA imposed by their employers). And at that scale, even a single person or a small group can make a successful game, given the motivation and resources.
It would be interesting to hear about a forum where dev discussion is happening. I only know forums about proprietary game-development. There's opengameart.org about assets.
Looping back to this, I think today most places of discussion are sort of agnostic towards libre/proprietary. The majority of folks you might see in an engine's community, or itch, or various social medias will be doing proprietary dev because it's the default, but there's nothing stopping FOSS folks from joining in--and I believe some do. As far as forums go I mostly have around LÖVE's and the Itch.io forums, mostly because they happen to be active and on-topic to my interests.
From my experience with those, if I had to put a finger on the biggest source of developer discussion...I think it's actually complete newbies? When someone shows up with no idea what to do and asks broad questions like "Hey guys how do I make a platformer", it tends to bring all the experienced folks out of the woodwork to discuss and argue about the best resources, techniques, etc...
On the design side, I've noticed that chatrooms tend to lend themselves best to those discussions. Usually it starts with people sharing whatever random bits they're working on, then inevitably people start to make suggestions on some of them and the back-and-forth can result in some really interesting discussions. I think some of the best design talk I've had was actually chatting in the community around a streamer I follow, who specializes in RPGs. Sharing a little bit of work I was doing with people who had a lot of opinions about the relevant genre got a lot of productive discussion about design and mechanics.
I'm not too sure where I was going with this tangent honestly, but maybe it'll interest someone who reads it.