(obviously that's just my own travel experience, I don't doubt there are places in all of those countries where that's not true)
The reason people try to control children's access to tech and social media isn't because they might see a nipple (lol) but because it's addictive and prevents them from doing anything else.
If we realy want to "protect the chidren" we should start by dismantling FAANGs.
If they are looking for porn themselves (e.g. are of the age to care for such things), they can download it themselves via Safari.
But generally speaking there’s no need to trade this stuff on iMessage.
I'd suggest arguments for or against any particular culture or lifestyle, or attempting to deny they exist, exceed the scope of discussing the feature itself.
Nudist beaches or naturalists' resorts are everywhere in Europe, even where it's less obvious, like in the former communist block.
EDIT: if we are talking about kids nudity, like changing them in the open, in front of everybody, that's never been an issue in my almost 50 years on this Planet as European living in Europe.
That aside, it would be nice if we could survive just a single thread about a child protection feature without endless absurdist arguments and strawmen being offered.
I appreciate the reply to this comment indicating nudity is often unsolicited. That's very true. A productive thread regarding a feature like this might include discussion of how to limit its scope, or how it might be safely extended to other kinds of harmful content a child might want to avoid. A pointless thread might be one where the heart of the tech industry continues to ignore the fact many new parents ban all unsupervised Internet and new media usage for their children well into their teens due to problems like this remaining unsolved for decades now, and the industry being utterly incapable of having a sober conversation about it.