trying to make the mace the way it works in 1.21

Presumably they're just using the fall distance of the player. I'm pretty sure there's a function to get fall distance, but if there isn't, you could pretty easily make one yourself. Make a player-persistent number variable that sets itself to the player's current y value whenever they're on teh ground, and becomes locked when they're in the air. Then, when they attack, you can get the difference between their current y position and their saved y position, and use this to determine the additional damage.