I’ve also just tried the script from Can't play media on Plex after opening with Apple TV source change - #3 by moralsnipe. It seems the Plex for Apple TV client is always Unavailable, even when it’s open and idle.
If I manually play something it will correctly show the status in Home Assistant, but as soon as playback stops it says Unavailable, and even activating the scan client button won’t change it.