This entire page sums up my frustration with Microsoft products and support as of late. Half-baked and unnecessary U.I. changes that feel like they were just made for the sake of change followed by responses from support that haven't updated their support material to reflect the change or is not at all related to the issue.
Then again, if Microsoft support teams are fully reliant on OneDrive, Sharepoint, and OneNote, their respective knowledge bases have not synced up and OneDrive is stuck "Looking for Changes".
To create more space for tabs or pages, my suggestion is to move the "search notebooks" function next to the Sticky Notes and Share buttons. There is plenty of space there at most resolutions and it will not hinder the viewing pane in any way.