There have been a number of historic events that had large measurable effects on human intelligience.

Invention of libraries

Invention of public schools

Building of large universities

Invention of leaded anti knocking compounds in fuel

Policies which reduce or limit access to said public education

Covid-19 longterm impact on cognitive ability

Policies which cause intellectuals to flee to other countries

Even if the average persons intellect is in decline, things are getting more complex. Society continues to advance in the long term, equity improves, crime and disease rates go down.

We're seeing bumps in the trends, not reversals.