The anectodal example in my country is the old (pre-WW2) math problems book that was used then and after war at middle education level (lycee). Later on the pr

The anectodal example in my country is the old (pre-WW2) math problems book that was used then and after war at middle education level (lycee). Later on the problems from the book could be solved only by university students (engineering etc.) and nowadays only by math students. This is anecdotal of course and there are still lots of bright kids eager to learn math today.