Undo and Redo were the only outlined text buttons in the editor toolbars, sitting beside icon tools for bold, italic and the rest. They are now the standard curved arrows, with the same hover label and keyboard focus behaviour as every other tool on the row.
The names have not gone anywhere: hovering or focusing either button names it, Edit → Undo and Edit → Redo still spell them out with their shortcuts, and Ctrl/⌘ Z and Shift Ctrl/⌘ Z are unchanged. On a phone the two shorter buttons leave more of the toolbar visible before it has to scroll.