I do editing and am required to send a tracked version of any Word document I edit. But when I'm on the go, I work via Chromebook, which cannot use the full app. Change tracking seems like a pretty basic feature to be left out of Word Online, especially when you go to all the trouble of putting a Review tab up there. If you can view edit history, obviously something is being tracked, so it shouldn't be that difficult to enable people to at least toggle the classic tracking mode on and off.