Not necessarily lost, but certainly diluted by the content produced later. The lack of AI or even sophisticated computing creates a further problem in keeping that content indexed in ways that preserve its relevance and allow it to be surfaced.
Another consideration is how suppressed previous knowledge was during the time the AI ruled over humans and how much was really lost during the Butlerian Jihad.
I'm not sure the Dune universe is fully elaborated from our days, but I would assume that every early human colony established by sleeper ships or embryo carriers would have a curated subset of human culture selected for maximising colony success. With that kind of redundancy, it'd be difficult to completely eradicate such information.