As a former staff member who took part in many "wtf is the VIP forum" threads, I'm all for killing this because it is -and has always been- strictly pointless.
I think that having a private discord channel would be more than enough. I don't even understand why the need to have a subforum anywhere (because it would be literally this 2.0) and having a "secret" thread in the Treehouse wouldn't be that much more useful since it'd only mean having the same discussions except with less people than if you simply posted the thread/made your post in the open. Don't misunderstand me, I get how some people (cough me cough) would feel more comfortable in a "small group" than in the open, but it's not like supporter is a particularly private group. And, at some point, there are enough people posting so that it feels like... the Treehouse. With only 70% the usual population. Again, I don't get the point. Make a Discord server (or a Skype group if you want to make sure nobody uses it ever again :P) and move on. And if there is enough demand, or if it can serve to soothe objections, open the "secret" thread because what harm it can do. At least it's something you can advertise in the Donations page, hah.
@the scrapbox idea: I think it's a good idea, I don't know why it should be supporter-only (probably anybody who has a good enough idea and can post about it in CQ&F deserves a fair hearing), and it certainly is going off in a tangent.
PS: Woo it felt good making a post like this.