This policy setting controls the default file format for saving workbooks in Excel. The recommended state for this setting is: Enabled: Excel Workbook (*.xlsx) Rationale: If a new Excel file 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 default 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.
Solution
To establish the recommended configuration via GP, set the following UI path to: Enabled: Excel Workbook (*.xlsx): User Configuration\Administrative Templates\Microsoft Excel 2016\Excel Options\Save\Default File Format Default Value: Disabled. (Excel saves new workbooks as *.xlsx but users can override this default.)