There are plenty of very long-term studies going on where groups are identified and then followed over years or decades.

Another approach is today you might go looking for people with different levels of airborne lead exposure as a child, and then compare the average income today to see if their is correlation etc. In these kinds of studies the treatment groups weren't wittingly part of the experiment, but you can look backwards.

Either way, in 20 years we'll probably be able to identify schools where AI texts predominated earlier than other schools and, adjusting for other factors, try and tease out the actual impact of that choice.