From what I was able to figure out from watching the episode tags, most of this reel was from Season 8 episodes. Most of the people on the show were already aware that season 9 was it and they weren't going to be rehired for Generation 5. One said plainly in one of the clips "What are they going to do? Fire us?" Hasbro isn't going to pay the voice actors from G4 for obvious pay requests, especially by Tara Strong and John DeLancie.
It will be interesting to see who all is hired for the next movie due out in Sept. '21, as announced today.