Undo typings is currently not a supported action using the z shortcut. A list can be found here: https://help.trello.com/article/1230-undoing-an-action-in-trello
If you are looking to undo a text edit, check if the browser itself has an undo feature that might help in this case.
You must be a registered user to add a comment. If you've already registered, sign in. Otherwise, register and sign in.