This is extremely naive and short-sighted. There is a literal example of this happening rn, and hopefully you will see why your approach isn't that good.

UK's OFCOM is currenly issuing legal threats to 4chan, for allegedly serving adult content and not willing to implement age verification. 4chan's lawyer tells them to pound sand[0], on the basis that 4chan is hosted in the US and has zero business presence in the UK, and UK is more than welcome to ban the website on their end through UK ISPs. The saga has been ongoing for a while, and the lawyer has been pretty prolific online talking about the case.

Anyway, following your approach, UK should embargo US over 4chan not willing to implement age verification as required by UK law? I plainly don't see this happening, or even being considered, ever.

0. https://www.bbc.com/news/articles/c624330lg1ko

4chan servers are in the US and the owner is in Japan. If the US wanted to they could seize all the servers but they will not because they have real time monitoring of all activity on the boards and have ever since Christopher testified before congress and the site was sold. If anything 5-eyes want that site to be unrestricted. 4chan has been a goldmine of people self reporting for wanting to shoot up or bomb places, as has Reddit leading to many body-cam videos of the site users and in some cases the moderators being busted.

The IP addresses are all captured by Cloudflare. It is literally next to impossible to post on 4chan without enabling javascript on Cloudflare or buying a 4chan-pass which leaves a money trail not perfect, nothing is but most mentally unstable people do not think these things through.

Should legislation be added to require the RTA header 4chan could and likely would add it in a heart-beat. They already have some decent security headers in place.