How about, you know, philosophers and other learned people who's life job is to study it?
I have mistakenly written "argument" instead of "algorithm" in my original post (edited now), which may have lead to a misunderstanding. Otherwise, could you please elaborate?
Another signal source might be based on the assumption that the important bits would be subject of suppression efforts, like topics that disappeared from school curriculum, internet censorship filter rules (for example [2]), takedown notices, banned books, topics journalists wrote about that were fired/defenestrated, wikipedia articles that were removed/renamed or heavily edited, unusual court cases with abnormal patterns might be recoverable from public records (like the Steven Donziger case [3]), websites on the internet archive that were taken down by infrastructure companies, there might be topics that are ignored by modern historians etc.
[1] https://meta.wikimedia.org/wiki/Research:Wikipedia_clickstre... [2] https://lumendatabase.org/faceted_search?sender_name=Europea... [3] https://en.wikipedia.org/wiki/Steven_Donziger