I have noticed that some ISO files contain this product.ini file and others do not. For example, if you build a version 1909 ISO file using the Media Creation Tool, it does have this file, but if you download the complete ISO file directly from Microsoft, it does not contain the product.ini file.
Still, as long as you can find a copy of this file, the primary generic Product Keys (such as for Windows 10 Pro, Enterprise, Home editions) do not vary from version to version of Windows 10. They have remained constant since 2015.