I’m on a forum of about 100 people who chat about crossover fanfiction, with a good share of us writing our own fics. And other random topics too, of course. It’s fun, self-hosted, and no news feed.
Maybe it can still exist because of the high proportion content creators, making the community feel like a community.
They're still around but the space has changed a lot and the forum software is outdated too. It's much cheaper to run servers now but most people have moved on to social or chat groups.
$5/mo is how Akismet spam detection costs.
You don't need to be paying for spam detection on a small community forum that is being actively moderated. Set up the software to prevent newbies from making new threads without moderator approval, use a DNSBL, have spam reports from anyone who's been around longer than a week automatically hide posts, remove said privilege from people who abuse it. I've never had a serious spam problem on a small forum I was actively moderating. Same with mailing lists. Also, if you do feel like you need spam detection, Akismet is available for free if your forum, like many, is non-commercial.
Additionally, bemusement - watching people go crazy over Reddit and Slack, when all I can see is glitzy, more newbie-friendly, centralized incarnations of Usenet and IRC.
Don't even get me started with Facebook (though, I hear FB isn't even popular with today's kids - rather, it's that thing old people use.)