Collaborative editing no longer allows you to discard your changes. It suggests that, instead, you "use undo (Ctrl+Z or Cmd+Z) to discard your personal changes".
However, if I'm editing a page, then I leave editing, and then return to editing the page, Undo is not available. The option is greyed-out until I make additional edits.
Someone on our team made some edits they'd like to revert, but they aren't sure of every edit that they made on the page.