Your suspicion is probably correct, and in terms of places you can go (online) to have a conversation that is asynchronous, long form, and comparatively private

Your suspicion is probably correct, and in terms of places you can go (online) to have a conversation that is asynchronous, long form, and comparatively private, I think it's all gone off the public-facing internet, at least for many categories of discussion. Slack, Discord, Matrix, probably some other platforms I'm not cool enough to know about. A long, long time ago, I ran a BBS: now I run a Mattermost server. Feels like about the same level of quality to me. The conversations there run the gamut from lower (more vulgar, less mature) to higher (more in-depth, lengthier, more thoughtful, taking place over a longer time frame) than, say, Twitter, Reddit, or HN. It's not hard to set up an invite-only, hosted chat server. It is somewhat harder to convince people to use it, because a lot of people don't want to run another app. If I had to choose today, I would go with a Matrix-based solution rather than Mattermost, but it wasn't available at the time. Most solutions, including Mattermost, lock you in by not letting you export the (full, including DMs) conversation archive, so you have to choose something you want to stick with for a while.