Excel-DNA-Compatibility-with-Excel

• Excel-DNA only works on the full Windows desktop version of Excel
• Both 32-bit and 64-bit versions of Excel are supported
• Basic UDF functionality work from Excel '97 on
• RTD (and related features like async functions) only work from Excel 2003+

A future version (probably released around 2019) will only support Excel 2007+, so that's a good baseline to support.