How does this statement relate to the study of history: “The animals saw no reason to disbelieve [Squealer], especially as they could no longer remember very clearly what conditions had been like before the Rebellion”?

This is a scary prospect, especially in today's environment of truth and post-truth. Governments can change history and textbooks to match their ideology. If enough time goes by, and people who know the truth die or are too afraid to speak it, the new version of history actually becomes the truth. If we do not record and learn from actual history, we are bound to make the same mistakes over again.