The super double jump is now universal. The old way of doing it was to hit 9 for exactly 1f for a double jump, but there were ways in making it easier, like buffering the direction in the hitstop of the previous move. This old way of doing it still works, and in addition, you can now just input 29 as a doublejump to get the sdj. If you don't want to accidentally get sdj during fuzzies, you can try holding 9 for them