I think they'll just send X-Is-Over18: Yes. Nobody actually cares about this issue enough to invest money in it - just enough to get certain stupid politicians off their backs. There will be third party browsers that always send the header, and they'll be banned from the app store upon discovery, and if they get famous enough their creators will be sent to jail, just like Tornado Cash.
This should work the other way: a website must send a header that the content in the response is safe for those under 18. If there is no header, the browser doesn't display the page. It is easy to implement and there is no need to change existing websites for this.
