To the best of my knowledge Plex has never been open source. It's just easy to mix up because it's been using Kodi as a base the whole time.

Indeed. They forked Xbox Media Center back in the day, to produce a Mac version when none existed.

Then they became a client/server model.

They are supposed to provide the source code for Plex Transcoder, since that's a modified ffmpeg build. In practice, the version they share is not up to date with the version they ship.

They are generally speaking, closed source.