As Comp mentioned, the backward step in HS is legal as long as it happens prior to the hands coming together.
It was mentioned that most pitchers who played more than HS wouldn't use the backward step because it wasn't legal anywhere else. This is what I had observed for a long time, however, I have noticed in the last few years that some pitchers are using the backward step or starting with one foot behind the pitchers plate in HS. It does have an advantage as it allows the pitcher to create some additional drive on their push off and could give them some additional speed.
I did an ASA tournament a few years back the week after HS season ended and I had to call an illegal pitch on the first pitch of the game because the pitcher stepped back....i got about halfway to her to explain what I called...she just looked and said... I know....HS season is over. She then pitcher the rest of the game perfectly legal.
I agree and there is a LEGAL advantage in LL and HS ball. Think about it for a second, how many DFPers use Java's 2-step and Pushback drill? When you do them don't you feel that extra momentum, your DD throws with some extra velocity, etc. etc. They are awesome drills. Now legally modify those drills into LL or HS ball and you have extra momentum IMO.
I understand why many are taught to do it one way (both feet on rubber) and it so they can pitch legally anywhere under any rule set. However, my experience is that a pitcher can easily make the adjustment if they have experience with Java's drills. And if they do, why not take advantage of it? Just my two cents.