New Outlook left navigation bar

Have you submitted feedback about this already?
You can also consider upvoting this similar request;
folder pane doesn't stay open -- terrible design! · Community

That said, New Outlook is still very much under development in various areas and has a long backlog and many things in planning. Therefore, providing feedback and upvoting similar requests is important to make sure priorities and expectations align better.

Also note that development within certain areas is often within cycles. This means that they for instance focus a while more on features, then on the framework, then on the optimization, then on the user interface, etc... 

The App Rail containing all the Outlook modules (Mail, Calendar, Contacts, To Do, etc...) and other apps will likely not go away. However, when Microsoft has sufficient data about common screen settings (such as aspect ratio, width and height in pixels, width and height in physical dimensions, dpi/scaling settings, etc...) they will look at how they can optimize how Outlook collapses and expands certain areas based on the window width and what the minimum size for a certain pane should be.

Right now, at 100% scaling, the window must be at least 918 pixels wide before the Folder List shows up with the Reading Pane on the right side. This could indeed quickly become an issue for smaller screens which have scaling set to 150% or even 200% and Portrait works differently than Landscape.

With the Reading Pane on the bottom side, the Folder List will always show, even with the minimal required window width of Outlook (646 pixels).