I read that Steam Greenlight turned into Steam Direct. Is the plugin going to work with this new system?
Steam Greenlight and Steam Direct were/are just the ways to get your game approved to be made available on Steam, it has nothing to do with what you use to make the game.
What does relate to the plugin is the Steamworks SDK, which this plugin does work with from what I can see.
I myself will be releasing a C2 made game on Steam in the new year and will be purchasing this plugin soon so I can set up Achievements and the like.