the difference being that it is very hard to avoid something added to the water supply, while it is usually possible to avoid the fortified varieties of food
Any filter removes it. Most people do that anyway by default.
I would venture to guess that is not true in the US overall. I expect more people have no filter on their tap water than those who have one.
You're probably confusing Fluoride which is actually quite difficult to remove, often needing expensive and complicated filtration such as Reverse Osmosis, with Chlorine which is trivial to remove with charcoal filtration.
I think you’re going to be surprised at how difficult it is to avoid iodized salt, whereas avoiding fluoridated water is as simple as buying a carbon or reverse osmosis filter (or buying filtered water).
It's as easy as using kosher salt. Read labels, child.
As long as you don't eat out anywhere; or in the case of other fortifications, best to bake your own bread.
Are you suggesting all restaurants filter out fluoride from the water they use?