I'm not familiar with this either, except I know there's a movie about how dumb the future generations become (forgot its title and haven't seen it). But this reminded me of a joke I saw:

Teaching Math through the Decades

1. Teaching Math In 1950s

A logger sells a truckload of lumber for $100. His cost of production is 4/5 of the price. What is his profit ?

2. Teaching Math In 1960s

A logger sells a truckload of lumber for $100. His cost of production is 4/5 of the price, or $80. What is his profit?

3. Teaching Math In 1970s

A logger sells a truckload of lumber for $100. His cost of production is $80. Did he make a profit?

4. Teaching Math In 1980s

A logger sells a truckload of lumber for $100. His cost of production is $80 and his profit is $20. Your assignment: Underline the number 20.

5. Teaching Math In 1990s

A logger cuts down a beautiful forest because he is selfish and inconsiderate and cares nothing for the habitat of animals or the preservation of our woodlands. He does this so he can make a profit of $20. What do you think of this way of making a living? Topic for class participation after answering the question: How did the birds and squirrels feel as the logger cut down their homes? (There are no wrong answers, and if you feel like crying, it's ok.)

6. Teaching Math in 2000s

If you have special needs or just feel you need assistance because of race, color, religion, sex, age, childhood memorie, criminal background, then don't answer and correct answer will be provided for you. There are no wrong answers.

7. Teaching Math In 2020

Un hachero vende una carretada de maderapara $100. El costo de la producciones es $80. Cuanto dinero ha hecho?

Ok, I apologize in advance if this joke offends anyone.

I've not seen any actual research on the subject, but just from my own personal observation and experience in my own country, the final year high school educational levels have certainly declined over the last 50/60 years.