Minecraft anarchy servers are pretty much the closest to how much the game enforces rules on you as they can be. Generally they’re free of most rules but the obvious cheating and hacking prevention. There’s no rules about griefing, pvping or anything else that you might fancy and most players take the most joy of simply being unlimited by anything but the game itself.
There are still plenty of players who are going to take time to build their fortresses and houses on such servers, but the rules are a bit different. You cant really protect anything when you’re offline, so most people simply hide the stuff they managed to mine and scrape together. Usually in a few dead drops so even if one of them is found out, they can still rebuild with others. Obviously there are also players who use most of their time looking for such caches to plunder, making the servers have a unique and fun dynamic.
The fact that the server admins don’t enforce any rules on the server does not mean that there aren’t any in effect. Most servers have their own set of rules, enforced by the players themselves which makes it all the more fun in my opinion. You mess with me or one of my friends, you’re going on the black list and you can be sure to be attacked by anyone who shares the list, even going so far that you might be hunted down and killed simply for playing. It makes the game more flavorful in my opinion.