Reddit, the place where it had a marked decrease in users once third party apps (that, amongst other things, hid the ads) were blocked? The place where half the users keep using the old UI because it gave them a sense of customisation that, again, hid the ads, but also felt to them like it was a place they had some ownership thereof?
In any case, I've never suggested these places aren't forums. I will suggest though, that you will *never* build the same kind of connections on any Facebook Group as you would on a classic forum, for multiple reasons (and only one of those is monetisation related)
As for the likes of Discord, you absolutely can build a community of people on there. What you can't do is find information that was posted 5 years ago. Or even, in my experience, anything older than a year in even a modestly quiet community. The forum I mentioned that closed over the weekend wasn't just a directory, it was a storage repository of styling bits and pieces, snippets, bits of functionality. All of which will now be siloed off in the best possible case or lost and need to be recreated in the worst - and in a community that brings in new people by way of them looking at existing resources and learning to participate by engaging and tweaking those resources, that's a fairly catastrophic loss. But they're not paying customers of anyone's so no harm done, just a bunch of silly people trying to have a good time with what's left of their miserable existences.
But you're choosing (again) to ignore the real point I'm making, and that's fine, because your stance and mine have never really aligned, let alone been compatible anyway.