Command + Z
Jon Gruber doesn’t think iOS has gotten ‘Undo’ right yet on iOS:
Undo has been in the same position in the same menu with the same keyboard shortcut since 1984. Undo and Redo are powerful, essential commands, and the ways to invoke them on the Mac have been universal conventions for almost 35 years. (Redo came a few years later, if I recall correctly.)
iOS does in fact have a standard convention for Undo, but it’s both awful and indiscoverable: Shake to Undo, which I wrote about a few months ago. As I mentioned in that piece, iOS does have support for the ⌘Z and ⇧⌘Z shortcuts when a hardware keyboard is connected, and the iPad’s on-screen keyboard has an Undo/Redo button. So for text editing, on the iPad, Undo/Redo is available through good system-wide conventions.
The shake gesture was fun and novel in the early days of iOS but it’s silly, inefficient, and cumbersome in 2018.