That said, outside of the merits of this approach, I am dubious of any actual implementation given 2 points.
1) Protecting the youths will always be a leaky bucket. With disadvantaged youths possibly more at risk. Those exposed to non-compliant parents ("cool" parents who are ok with sharing unsuitable content) or lacking strong parental involvement, likely won't benefit a great deal from any implementation.
2) Anti privacy social networks stand to gain the most from targeting ads utilizing signals from most child safety acts. They also might be able to reduce some costs from moderation if they can make it someone else's problem. I'd argue the net social impact from these social networks is likely both more normalized and strongly negative for our youths than any smut.
On the balance I'd say we are better off investing our energy in other places.
Or simply parents who won’t agree with the government on what is suitable for their children.
We already have parental control on all mainstream operating systems, why cannot this simply be the responsibility of the parent as are so many other things regarding what children do, watch, eat etc?
Adding a header to a web server or load balancer or app server if done globally can be done in a minute or two. Maybe 5 minutes for the intern not counting QA testing.
But you are right, the inverse is easier. I like that too. That was debated in the other recent thread.
None of that is true. Not even close. It will work if there are laws with big teeth as the government can have contractors that scan for the header. Header missing? Adult content? User provided adult content even if it violates the rules? Well darn, there goes 10% of revenue. Also it is not "this is a porn site". RTA stands for restricted to adults. That can include literally anything that is not appropriate for small children including but not limited to social media, forums, gallery sites meant to be SFW but users content is not pre-moderated meaning people can see it before a moderator does. What Restricted to Adults ultimately means will depend oh the laws behind it.
An actually useful standard would need to be much more granular, and be designed as a whitelist not a blacklist, as I and previous commenters said.
[1]: https://wiki.whatwg.org/wiki/MetaExtensions#:~:text=Restrict...
Here [1] is the only external link that is relevant and don't take their banner out of context. This was originally by the adult entertainment industry to self label and self regulate but is now used on any site that is not appropriate for minors. Entertainment could be inappropriate humor for example. That does require porn. I would link to some inappropriate humor but that would be inappropriate.
"The RTA Label is free to use, voluntary, and universally available to any website that wishes to clearly and effectively label itself as being inappropriate for viewing by minors."
Parents today can accomplish what you are suggesting by installing parental control software and only allowing access to things they explicitly approve.
This can also be done via headers explicit blocking of all the things and was suggested in another thread. [1] Some people liked the idea.
[1] - https://news.ycombinator.com/item?id=47952999