As of December 26th, 2016 Pro.photo will no longer sell their proofing plugin which worked with Prophoto 5 and Prophoto 6 versions of their website. It was announced that support for the plugin will continue for each version of Prophoto until those versions are not supported. For prophoto 5 that would be with the release of Prophoto 7, and for the Prophoto 6, that support would stop with the release of Prophoto 8.
Long story short, the proofing plugin required a great deal of work that wasn’t offset by a greater amount of revenue. But not to put words in anybody’s mouth here is a link to the Press Release from the Pro.Photo.