I'm well aware of those possibilities. The two biggest problems with them are that 1) they still apply to everyone, rather than only to those who opt into them and 2) governments and companies are in practice going to push for the versions that identify people and provide more information.

If you make it possible for governments to decide what content is "limited to adults", they can and will abuse that capability. "Porn" is the battle cry, to make it uncomfortable to argue against; often, other information the government wants to restrict becomes a target. The only way to prevent that is to deny the capability in the first place.