May I know what is the oldest Excel version that being supported by Outsystems?
You mean for importing data from an Excel using Excel To Record List? I would assume that any valid XLSX file can be imported this way, but I doubt old XLS files can be read (they're a totally different format, deprecated in 2007).
Thanks @Kilian Hekhuis would you able help to confirm Outsystems is unable to support XLS file format?
That would be easily testable by saving an XLSX with Excel into the old XLS version, and trying whether that works.