I know that a pitcher must start with both feet inside of the 24" plate. Is there any rule where her pivot foot must land?

Rules of gravity would say on the ground.
Welcome to the board.

The pivot foot can't land anywhere and be a legal pitch. The pivot foot must be in contact with the ground and dragged away from the plate by the forward motion. If the pivot foot lands, you had a leap.

If you meant stride foot that must land within the "24 inches. Many pitchers when throwing a curve stride outside that area and need to be called. The plate umpire is the one who can see this. With all the other things that we try to see it needs to be obvious. I have called this and coaches don't like it much.

Ive called it too and the obligatory coaches warning that goes with it.