I guess “temporarily unavailable” is in the eye of the beholder. 
We were told Plex Mix would be missing for a while in this announcement at the end of June, 2019 - and then there was a glimmer of hope when track moods were re-enabled for the server in mid November. Two months later, you still can’t edit the moods or even see them unless you open the XML for the track, and then you have to trust that the scanner/agent has actually added that information. 
Sometimes, what you want to hear is “Hound Dog” by Elvis, and then continue on with similar music. Clicking the Elvis Presley Artist Radio button gives you no guarantee that “Hound Dog” will ever be played.
And even though you can find my version of Hound Dog here at MusicBrainz, and find it at All Music with associated moods and styles, neither the album or track has any of that information.
No wonder there’s no Plex Mix.
