Seems like a case for HTTP 451 (Unavailable for Legal Reasons) rather than 404.

HTTP 666 (We're evil) seems more fitting here.

It looks like the issue was that, in Italy, copyright expires 70 years after the death of the author or the first translator of a work.

PG works based on US copyright law. And as I understand it that's also 70 years after author/translator death. My gut feeling is that if anyone tried hard enough this ban could probably get lifted

That’s only the case for works published after the mid-70s. For works published before (which is all current PD books in the US), it’s 95 years after the date of publication, with a few exceptions where people failed to file renewal notices.