I've seen articles on that. What I do not like about that is one has to trust that is really the way the system works and that special people do not have a special API key to get their own hash from the adult site related to a user ID and then submit that has to a special API end-point to reverse or undo the anonymization. Having been a liaison to law enforcement I just assume that is a thing but I am also fine with people saying I am paranoid. A header does not require this level of trust nor a dependency on a third party see recent Cloudflare outage.