Even if you use a fluoride-free toothpaste, you are most like still being exposed to fluoride in some shape or form. If it's in your municipality's water supply, you're taking in fluoride, even if it's filtered. I did some research on water filters for pitchers, and also filters on faucets. Those filters do NOT remove fluoride. And a lot of bottled water has been third-party tested, and the results mostly showed fluoride is in that water also.

I could be on the wrong track here, but possibly the reason we cannot filter out "fluoride" is that it is atomic in size.
From wiki:
"Fluoride (/ˈflʊəraɪd, ˈflɔːr-/) is an inorganic, monatomic anion of fluorine, with the chemical formula F− (also written [F]− ), whose salts are typically white or colorless. "
Now, this definition of "fluoride" is also thought-provoking.
Because an ion cannot or does not exist alone.
So why is it called "fluoride" as if it were not an ion?
To be stable, a "Fluoride" ion combines with a negatively charged ion to form a salt. If "Fluoride" is in water, it probably exists as an ion where its negatively charged partner is also in the water.
Just wondering how it works. Corrections are welcome!
Also why have THEY said "Fluoride" is good?
I remember my dentist gave a "Fluoride" prescription for my children to take daily.

Just spitballing here; sodium fluoride is what is generally added to toothpaste. So it is a 'salt' in this form. And a larger molecule, which could be filterable.

Good thought and actually a covalently bonded organic molecule could be large enough to filter out, but ionic salts can be extremely small, unless one of the ions in the compound is an organic polyatomic ion.
Sodium fluoride is small, just two atoms.
I do wish I could find a scientific analysis about this.
It's been a really long time since I studied Chemistry!