Meta-analysis study from China says fluoride in drinking water lower IQ by 7 points on average in children, with more fluoride causing higher effects [1]. Probably politically infeasible to do such a study in the US because of left-right polarity but I'm sure once it gets widespread acknowledgement in a couple of decades, there'll be people saying how it was obvious all along.
Other studies too from a quick google search. Pregnant women drinking fluoridated water lowers children's IQ by 3-4 points [2]
1. https://www.hsph.harvard.edu/news/features/fluoride-children...
2. https://www.forbes.com/sites/brucelee/2019/08/20/fluoride-an...
From google
https://pubmed.ncbi.nlm.nih.gov/18695947/
https://pubmed.ncbi.nlm.nih.gov/31424532/
https://www.ncbi.nlm.nih.gov/pmc/articles/PMC5285601/
This is a good article as an overview of the problem: