I'm not in the office right now to verify what/why this is here in the release notes for 2013 SP5 to be quite honest. I can tell you that there was a mirror option added in SW 2014 "Bounding Box" in which gives the more desired result in many scenarios rather than the "Center of Mass" which was the default and only only option prior to 2014 when the "Create Opposite version" was not selected because the mirrored component was really just a repositioned/rotated component.
I can assure that no options were removed in 2013 SP5 that you originally used. I think what may have happened is the UI for the 2014 option was "let loose" sort of speak in the 2013 SP.
Thanks Don! I appreciate the quick answer.