Earlier quoted context omitted.
The biggest defederation example I know of (with the caveat that I haven't actually used Mastodon in years) is that when Pixiv launched their own instance it quickly grew to be something like the 1st or 2nd biggest instance in the network and was eventually defederated by many other instances because people were REALLY antsy about the fact that a lot of Japanese art on Pixiv is illegal in other countries.
"Japanese art" is kind of underselling pornographic images of fictional children. Most forms of Japanese art, I presume, aren't of this nature.
fwiw while the underaged illustrations are the most objectionable part, there's also other stuff that could get you in trouble - for example, illustrations of ww2 nazi soldiers with accurate garb would run up against regulations in germany and put the operators of german fediverse instances at risk. There are a few high-profile japanese games that feature accurate ww2 uniforms, weapons, vehicles etc so if you end up with fan-art for those games on your service you will quickly need an answer for "what happens if content on the fediverse is illegal in germany"