Excel 2003 Xml listed here: https://libreofficehelp.com/libreoffice-calc-supported-file-formats/
But it does not appear on the list when saving.
Version 6.2.
This does not work with Save or Save as .... Select File>Export .... In the dialog box “Export” choose “File Type” “Microsoft Excel 2003 XML”, enter the desired file name and click OK.
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With me Windows 10 Home; Version 1903; 64-Bit | LibreOffice, Version: 6.2.4.2 (x64).