I have owned and showed walking horses. I showed "lite shod" and not "built up" but was still around many barns in Tn. / Kentucky that train all types of gaited horses, and I have riden them all. Yes, I am aware of those that "sored" horses, used chains as action devices etc.
Having said that, I would add that, IMO, we don't want to get too smug on the forum about horse training methods. It is pretty much an acceptable practice to shock a dog that is running too big to make them come in, shock them to make them come to us, shock them to make them whoa, etc. We may change the words from "shock" to some other less offending word, but it is the same feeling to the dog. Yes, there are lower levels of stimulation, like there are lighter chains.
I , in no way, support soreing horses. It is just plain wrong. But we need to remember what we do in the name of training as well.