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.
