Hey db0, I think this is a great idea. But maybe give them a bit more time to grow. Like instead of 1 week, maybe allow 1 month from creation date. If there are no posts/users for the last 2-3 months than give them warning and if nothing changes remove it. Also having a a list of purged communities and reason why is good. This will give others a chance to see what is going on and why. And maybe eliminate clones.
Most if not all lemmy instances log at least ip address and username it is connected to. Unless instance owner specificly said that they are not, you are to assume the do at all times. In most cases this is to prevent any abuse on the server. So its recommended that you always use encrypted tunnel with any piracy activity.