I've said this before. The UK online safety act if they enforce it hard against fediverse instances, it will be the end of federation, for UK users without a VPN at least. Because it puts too much on the shoulders of small site operators.

In this case though, the exception most countries have for site operators to avoid being responsible for their user's posts is usually reliant on action being taken when content on your site is reported to you. There isn't really an exception for saying "Umm, wasn't from my site mate. Go follow the trail and get the original guy". The argument will be, the site you control has the content, remove it.

In the UK in the 1990s there was a court case [1] that might even form the part of the case law behind the publisher exception. In that case the claimant stated that the ISP was alerted to forged usenet articles (usenet was pretty much a good analogue for modern federated content) that he believed defamed him. They did not remove the articles (presumably because they did not originate on their usenet server, by their users I am not sure). He sued them and the court ruled in his favour. There's more nuance, but the take away is pretty much what we got in the law created later.

Since then we have enacted the Defamation Act 2013 [2], which has section 5 that gives SOME exemption to operators of websites that allow posts by third parties (that pretty much covers the fediverse). That makes it clear that if the claimant cannot identify the user (which would be the case for 99% of threadiverse users), and if you are informed about the content and do not take action, then you may be held accountable for the defamation. Now that just means that if they tell you X post is defamatory or should be removed for another legal reason, if you refuse to do so in a reasonable time period, then you can be held responsible and treated as the publisher of that message. So if it were to breach some law, they could sue you for it as if you posted it yourself. Which is kinda why I'd say just remove it if there's any doubt at all. I'm not a legal expert, but that's how I read the act.

I'm not sure how it works elsewhere. I live in the UK. But generally the rules are somewhat similar.

[1] https://en.wikipedia.org/wiki/Godfrey_v_Demon_Internet_Service [2] https://www.legislation.gov.uk/ukpga/2013/26/section/5