> What's needed is a distributed, decentralized system, ideally at the protocol level. Imagine if a GET request by default gave you the "current" version of a page, but you could send an extra header that said "give me this page, as it appeared at date-time X".

Sounds like Freenet USK's (see https://freenetproject.org/understand.html, search for USK (and boo on them for not having any anchors on that page)).