American IQ scores have rapidly dropped, proving the 'Reverse Flynn effect'

When IQ tests were invented folks didn't know about tests, at least in the US. They were rural immigrants who could maybe read. So when asked logic questions, they would answer pragmatically and be 'wrong'. That had some impact on perceived early low results.

As folks became better-read and educated they began to understand that IQ test questions were a sort of puzzle, not a real honest question. The answer was expected to solve the puzzle, not be right in any way.

E.g. There are no Elephants in Germany. Munich is in Germany. How many elephants are there in Munich? A) 0 B) 1 C)2

Folks back then might answer B or C, because they figure hey there's probably a zoo in Munich, bet they have an elephant or two there. And be marked wrong.

That theory could be plausible, except Flynn used results from Raven's Progressive Matrices, which is just pattern recognition. There are no questions about elephants or text-based questions that could introduce cultural bias. It's simply picking the shape that matches the pattern presented in a grid.

https://en.m.wikipedia.org/wiki/Raven's_Progressive_Matrices

I've often heard from humanities academics that STEM majors do not confer critical thinking skills.

Have you, or is this a very uncharitable gloss on what they actually were saying?

Yes, many times, from many of them. Google "stem majors lack critical thinking skills". To drive the point home, google will autocomplete it for you.