Comment by Quarrel

I looked in to this recently.

Turns out that carbon filters do filter fluoride, but only on the first few X litres of water, where X is in the first 0.01% or so of their expected lifespan. So, they do, but not really usefully in any sense.

My filter that I wanted to know about (so that my kid is getting fluoride) is a 2-stage filter, with the other stage being a particle filter, but fluoride is very small and unaffected.

Your filter might be different of course.

FWIW, in my city, the water has essentially zero fluoride if it isn't added, and it has been a great intervention.