As I recall, this wasn't really something ever considered a conspiracy theory. It was something the authorities didn't figure out quickly when they should have, then REALLY didn't fess up to it, and covered it up and lied about it pretty horribly for way too long. (There was a very similar saga in Canada, BTW). So perhaps an actual conspiracy in some ways, but not really an example of something with a "conspiracy theory" crowd on the other side.

There aren't that many things that were generally considered conspiracy theories at the time but turned out to be valid.

City water fluoridation might be closer to being an example, perhaps? It wasn't a communist conspiracy, but the "not very good for you" and "not sufficiently tested" parts of the conspiracy theory crowd were arguably ultimately not wholly without merit. There still isn't a fantastic consensus, but those who were vehemently opposed at the time were not necessarily nuts, despite being seen as such at the time. These days their best observation is that it's definitely a medical treatment with an ingested substance, so it really should have gone through all the tests and approvals of any other ingested medical treatment to be assessed for benefits and risks, not to mention informed consent.

There is still a lot of shrill stuff about the subject (amazingly) but it's not so bad if you stick to web pages ending in ".gov".
Random one: you almost certainly get an unhealthy amount of fluoride if you live somewhere they put it in the water, AND drink tea, even without fluoride toothpaste.
ncbi.nlm.nih.gov: Risk Assessment of Fluoride Intake from Tea in the Republic of Ireland and its Implications for Public Health and Water Fluoridation
Tea is a super-concentrator of fluorides.

Jim