I have not read it but a review of “The Gymnasium of the Horse” by Gustav Steinbrecht - which is on the USDF trainer program reading list - has some unsavory quotes.
“The jab with the spur is the strongest and most emphatic influence with the spurs. It gives the horse a momentary, intense pain and, by injuring the skin, causes infection and swelling of the parts involved so that, for some time, the sensitivity at that point is even greater.”
Just because something is “old” or “classical” doesn’t mean it is good.