To use old Ductpost (.opt) style posts with PowerMill 2017 and later you need to download and install the file labeled DuctPost_2015_2016 from this page: https://knowledge.autodesk.com/support/powermill/downloads/caas/downloads/content/powermill-2016-release.html
You will still get the warning about them being no longer supported but they will continue to work for the foreseeable future.
As these posts are no longer supported it is highly recommended you get in contact with us about converting them to the newer Autodesk Manufacturing Post Processing Utility format (.pmoptz)