Answer
For the web-based Office client, clearing the browser's Excel-related cookies and browsing cache and refreshing resolves it; for the desktop Office client, close Excel and clear the contents of the local Office WEF cache folder (%LocalAppData%\Microsoft\Office\16.0\Wef) before restarting Excel. Alternatively, if the add-in was sideloaded through a Trusted Add-in Catalog pointing at a shared manifest URL, simply removing that catalog URL entry from Excel's Trust Center settings removes all add-ins sourced from it in one step.