Is the on_click property for panels not part of Lite-C? It seems to have disappeared from the current manual. If it is not in Lite-C, it seems like it should still be in the manual until C-Script goes away.

Yes you are right.
panel.on_click is no more documented (at least I could not find it in my A7 manual)

"on_click" in Lite-C is now a global event when you click somewhere inside the view with the left mouse button but do not hit any object or panel.

"event" is now the pointer to the function which is executed when clicking on a panel.

It was renamed to "event" because it's intended for other events too in the future, like right click and touching.

Three cheers for that! I've always wanted more panel events. The other thing that would be great is a variable like mouse_ent for panels, so you can know if the mouse is over a panel or not (and which one it's over).