I inadvertently removed the "flagged" shortcut from my Mail sidebar. How can I restore it? There is no "+" button under favorites in this version of Mail
How to restore "flagged" to Apple Mail sidebar in Monterey
Hover over "Favorites" and click the plus button. There should be a popup that allows you to add a mailbox. In this case, the mailbox is Flagged, which will be available from the picker in the popup. Also, as far as I know, the Flagged mailbox is not included by default in the sidebar.