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.