Again, RTA header doesn't work because it's a blacklist, not a whitelist, and also because it's only granular enough to say "this is a porn website" and nothing else.

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.