It kind of makes me think of how odd it would have been if many of the old forums named themselves like bookclub.phpbulletin.com, metalheads.vbulletin.net, or something.

There were hosting services for forums in which free or cheap tiers were on a subdomain like that. I think what you're asking is the inverse though, e.g. mastodon.alostinquirer.org. I find that odd too, as it creates a weird situation if the person hosting it wants to use different software for the same purpose, e.g. Pleroma or Firefish for microblogging instead of Mastodon.