How are you counting active users? I've checked the subreddits I enjoy and none of them have more than 1k users active right now. Given default subs such as worldnews are showing 33k active users right now, I can assume you're measuring it differently than I am (as the way I'm measuring has varying results with time).
I also wonder how many active users Lemmy/Kbin instances/communities/magazines have. It would be interesting to know to get a feel for how the amount of active users correlates to the overall feeling of community.
As far as I can see it existed and still exists though, it's just that there is less incentive to produce this kind of content and leave it open. I can find pieces of it still remaining, but the environment that favored this kind of content is gone. It's a natural result of how the system changes as people learn to game it and find ways to gain power or make it profitable. I hope the advent and growth of places such as this one gives it some fresh air, though.
There are modern websites that still bring those feelings to me. For example, this blog has impressed me with its content and creative visualizations: https://ciechanow.ski/ Personal websites and web forums just don't surface anymore when searching the web or browsing large communities/aggregators, but I can find them on places such as https://curlie.org/ (A modern-day web directory) and https://search.marginalia.nu/ (a search engine that focuses on non-commercial websites).