It's probably not quite as exclusive as you're imagining (certainly not as much as your 256 max 'CTOs 1'), but we're kind of in one aren't we? It can be done, it just takes substantial effort from a dedicated moderation team.
I helped moderate a traditional forum (Computer Juice, née The Computer Forums) in the late 00s maybe slightly into 2010s, I don't know how 'rich and insightful' it was, that wasn't really what it was about, but it served its purpose for a handful of regulars and guests that dropped by one time for part selection advice or because they had a 'blue screen of death', nothing like HN traffic I'm sure but that too would've been overrun with spam and ads if not for a few of us keeping on top of it.
You can't just set and forget, not with open membership anyway - and something like WhatsApp would probably always struggle with that because you don't have a great way to give much prominence to something Stripe CTO (for example) says vs. the keen doe-eyed cofounder CTO whose also currently the entire engineering team but hoping to make a first hire and talking the talk, sharing lots of random Medium articles on hiring practices and building hypergrowth teams etc.