I don't think they purge the archives, I think they just don't serve them on the wayback machine.
[1]: Sure, some people just want to hide embarrassing or incriminating content, but there’s also cases where someone is being stalked or harassed based on things they shared online, and hiding those things from Archive users may mitigate that.
I don't think it's mentioned in an official document, but it's usually referred to as "darking".
It probably safe to assume that the same concept applies to the Wayback Machine as to the rest of IA.
Edit: Here's a page that indirectly conveys some information about it: https://archive.org/details/IA_books_QA_codes