I keep hearing this from various people who aren’t dentists.
I am a person who is not a dentist. My understanding is that the medium bristles are not bad for your teeth, but can hasten gum recession compared to softer bristles. I have been told this by people who are dentists.
My dentist has told me to always use soft brushes, for the reason that using harder brushes + using the wrong brushing technique has caused the gums to pull back slightly from my teeth, exposing parts of the tooth that shouldn’t be exposed. That means drinking hot or cold beverages hurts like hell.
You should probably ask your dentist what’s best for your teeth, though.
A few years ago they pushed very hard brushes. We all switched to them. You can thank my generation for proving that hard brushes harm the gums.