I'm not disputing the impact of lead poisoning - as you've mentioned, it is well established in science. There are no studies, however, on how it has influenced the suspected contaminated areas both before and after it's removal. It's a possible reason behind falling crime rates, but there are billion other variables out there which could be just as influential. Citing the theory as if it was some obvious fact is simply un-academic pseudoscience.

The point is that our modern society has greater tools to deal with criminal behaviour, which can be an effective deterrent, and they have been recognised by researchers looking at pre and post digital age crime statistics/history as being influential.