My wife comes from a country that did not add fluoride to the water. She has very good dental hygiene but here teeth are not in good shape for her age. That's one data point that I have observed.
Her home city of Hanoi doesn't have tap water that the locals like to drink. Even after boiling I think it can taste bad. So she grew up with heavily filtered water or bottled water. I would guess her diet was lacking in many trace minerals from this.
I’ve noticed the same thing living in Taiwan. They don’t fluoride the water, and in general people have pretty bad teeth with caries and decay that I never see in Americans in their 20s and 30s. The highly sugary diet probably doesn’t help either.
As with many things genetics plays a big part.
I neglected going to the dentist for 15 years and take just ok care of my teeth (brush at least once per day, don't floss).
I was expecting bad news, lots of cavities, but my teeth are perfect. After some plaque removal, not a single cavity.
Asked the dentist about this, he shrugged and just said "genetics".