I'd like to second all of the remarks about the continued decline of customization choices in Office. This new space-waster on the left is just the latest example of no-opt-out changes that have been degrading my productivity since the turn of the century. People who need to work hard in Office apps need to be able to customize as much as possible to help minimize wasted mouse movement. Even some of us with very large desktops need to maximize real estate rather than look like a phone app. In an ideal world, all of the panes and ribbons would have a spacing slider so we could choose just how tight or loose we wanted the layout to be.
I was at my productivity peak the version before the ribbons appeared. I had custom menus with keyboard accelerator settings that enabled me to fly through work. I even regularly used custom floating toolbars to save the wear on my narrow-carpal-tunnel wrists. Now it feels like my productivity has had two decades of Death by a Thousand Cuts.