Information
This policy setting controls the default file format for saving workbooks in Excel. The recommended state for this setting is: Enabled. By default, when users create new workbook files, Excel saves them in the new Excel format. Users can change this functionality by clicking the Office button, clicking Excel Options, and then selecting a file format from the Default file format list. Disabling this setting allows users to choose from any of the available default file formats. If a new workbook is created in an earlier format, some users may not be unable to open or use the file, or they may choose a format that is less secure than the Excel format.
Solution
To implement the recommended configuration state, set the following Group Policy setting to Enabled. User Configuration\Administrative Templates\Microsoft Excel 2016\Excel Options\Save\Default File Format Impact: Enabling this setting does not prevent users from choosing a different file format for a new Excel file, and therefore, it is unlikely to affect usability for most users.