See you highlight the issue yourself ; forcing the websites to expose data allowing an initial set of restrictions. Once those are in place extending them beyond the initial « doing the parent’s job » scope is trivial. The infrastructure will be available to block any content.

Once those headers become mandatory to deliver content the days to http freedom are over.

Eventually the system will enforce the correctness of such headers in a way that piracy will be added. Then assisted death and lgbtq topics; I saw propositions along those lines already.

I remember one of those discussions involving European isps representative ; who is to decide on those tags and which to block? Why would 16 yo not being able to discuss a topic or reach out for help on sensitive topics? At what age are they individually mature enough for those topics ?