mac users:

You download a techincally-oriented GUI app with a plain text config file format (let's say KDL or such) that you're encouraged to open. You expect it to be located in...

I'd love to hear thoughts on this in the replies

~/Library/Application Support/com.vendorname.prog/
~/Library/Preferences/com.vendorname.prog.file
$XDG_HOME/prog/
somewhere else
Poll ends at .
@mattly old Mac me would have said Preferences, but modern apps seem to prefer to be under Application Support.