Your information is not quite accurate.
A combination high/low block is illegal. What changed was the stipulation that the low part is now illegal even if it happens simultaneous with the high block. Previously, only if the low block was delayed is this a chop block. Now any combination of high/low is illegal.
High/high is and always will be fine. That's just a double-team. Low/low blocking is also fine as long as meets all other requirements of legal blocking.
What I don't like is how the NFHS defines high and low, with regard to CHOP BLOCKS ONLY. High means above the knee and low means at or below the knee. So technically, a double team where a lineman is engaged with arms and shoulders, while a teammate cuts the defender in the thigh is legal. I hate that. I know of two broken legs on that exact block and, by rule they were legal. I'd like to see low as defined as below the waist.
I mentioned this is regarding CHOP BLOCKS ONLY. I hear a lot of chatter form the sidelines and even players on the field who don't know what a chop block is. By definition, a chop block is a double team. It requires two blockers -- one high and one low.
A single low block is called a cut block. The threshold on legal cut blocks is whether it's below the waist.
Cut blocks can be legal.
Chop blocks ARE NEVER legal.
To legally cut block, the blocker must be on his line of scrimmage, the defender MUST ALSO BE ON HIS LINE OF SCRIMMAGE at the snap. No cutting of linebackers allowed, and no cutting by blocking backs or lead blockers is allowed. The block must be in the free blocking zone. The free blocking zone disappears when the ball leaves it, and it only extends 3 yards behind the line. A team in shotgun formation loses the FBZ so soon that only the initial charge is legal below the waist. Any step back and delay make an illegal block. A QB taking a hand-to-hand snap buys his linemen a little more time because the FBZ might last a couple more seconds.
It's possible for the defense to be flagged for cut blocks. I've seen defensive backs take out lead blockers with low blocks. The rule just says no player shall block an opponent below the waist, except when the right conditions are met.
