The iPhone has an undo function, but it's not entirely obvious how to access it

If you delete or type over something by accident, just shake your phone

Hit the undo button and the previous text will be restored

To go back to the text you had before undoing, shake again and press Redo Typing

You can disable Shake to Undo in Settings - click General, then Accessibility

Scroll down to Shake to Undo and turn it off

You can even use it to undo autocorrect errors

When you see one, just shake to revert to what you wrote originally