Ttrpg.network seems to work well. As does the Star Trek one, even despite serious problems with some of their communities' moderators that the admins have failed to take action on.

I think it's a format that can make sense especially if there's a broad range of specific communities around a central topic. Like ttrpg.network can have communities dedicated to each RPG, one for memes, for art, for broader conversation about the hobby, etc. It means you know if you want something RPG-related, that's the instance to look for.

In a way, you could even say all the various country instances, including my own home insurance, are doing the same thing. What is a country instance if not "entire instance devoted to one area"?