"The NTP monograph concluded, with moderate confidence, that higher levels of fluoride exposure, such as drinking water containing more than 1.5 milligrams of fluoride per liter, are associated with lower IQ in children"
This is from an NIH meta-analysis. Its a pretty rigorous study.
https://ntp.niehs.nih.gov/whatwestudy/assessments/noncancer/...
"It is important to note that there were insufficient data to determine if the low fluoride level of 0.7 mg/L currently recommended for U.S. community water supplies has a negative effect on children’s IQ"
This is why there's such a fierce debate. Based on the most recent scientific literature there seems to be evidence of a dose-dependent effect of fluorine levels in water and lowered IQ in children, meaning it has some kind of neurotoxic effect. But we don't have robust evidence to say 0.7 mg/L has a similar effect. That doesn't mean it is DEFINITELY safe, it just means more research needs to be done and the current research that does cover the 0.7 mg/L range may not reach statistical significance.
The fact though the NIH suggests 1.5mg/L is likely unsafe, which is only 2x what America's tap water contains, I would not blame people for being uneasy about. It is often the case the that the FDA regulates food additives that have potential negative side effects to be limited to concentrations 10x lower than what is seen as unsafe.
I am not suggesting it's a straightforward choice to defluorinate water, but I see people often repeating claims like you have that dangers of fluoride are "thoroughly debunked" and that's simply not true. I don't blame people for having that sentiment either, because 0.7mg/L is seemingly still considered safe, and some of the loudest advocates of defluorination have no shortage of thoroughly debunked crazy views on things (possibly due to brains half eaten by worms). It makes it very easy to brush off the skepticism.
But it's also important to keep in mind science is built on the premise that one must be ready to re-evaluate past assumptions when new data arises, and generally speaking the new data around fluoride I hate to say seems to show there is indeed smoke.
It's also the case that when the US initially fluorinated water supplies it was a massive public health success, but these days it seems to make a much lower impact now that fluoride toothpaste use is ubiquitous (plus the levels were lowered from 1.0mg/L in the 70s, likely reducing its overall effectiveness). It is IMO both very reasonable to fund more research into this to know conclusively if 0.7mg/L is indeed safe, and also consider public health policy that focuses on promoting dental hygiene through other means in places that do defluorinate.
I do not agree with Utah's decision here mostly because it seems to neglect that defluorination will create a void that requires other public health policy efforts to fill it, poorer and less educated communities will suffer unless government led efforts to promote and make dental hygine affordable are not also undertaken.