Found the answer I was looking for. On iPad there is a built-in Undo button in the lower-right side of the keyboard window, but it’s only visible when you switch keyboard from letters mode into numbers mode.
Just making a plug for the iPad Pro keyboard, command-z works as expected.
a bit late but maybe useful for somene: there is a native three finger touch gesture that opens a native toolbar for copy/paste and undo/redo operations. So just touch your screen with three fingers.