There's a similar phenomenon of terminology creep with language terms like 'High German' and 'Low German'. This nomenclature has been adopted into fantasy literature to suggest a more noble vs. fallen version of some ancient dialect, but all it meant originally is that 'high' speakers were upstream (and therefore uphill) from the 'low' speakers.
This is one of those 'common misconceptions' where I hear about its refutation more often than I encounter people who believe the supposed misconception. Much like anything talking about map projections, in fact.
You may need to read more crappy fantasy books. To cite a completely representative example, consider 'High Valyrian' from Game of Thrones https://gameofthrones.fandom.com/wiki/High_Valyrian, a liturgical language that of course is contrasted with Low (or Bastard) Valyrian, a degenerate family of dialects.
I don’t doubt they exist, but the only two examples I ever hear are the GoT one you mentioned and High/Low Gothic in WH40k. But I see that factoid about High/Low German repeated multiple times a year. It was a common entry on ‘fantasy cliche’ lists even in the dial-up era of the Internet.
