I don't know the contract details of the chess.com bots, but I know that many of them are only granted for a certain length of time. This is what happened to the Beth Harmon bots. I'm almost certain chess.com could keep them longer if they could, but I think there are some contract/copyright nuances for why they can't keep them on chess.com indefinitely.
As for Mittens (and all the chess.com "personality" bots), it is just a version of Komodo because chess.com owns this. That means that if you sort of want to play Mittens, then just play the Komodo bot at a high level and get crushed by it just like Mittens...and you can even randomly recite the voice lines many people probably have memorized by now. xD
p.s. In the video below, Mittens and Komodo: They are the same picture ![]()
like this to vote or comment, maybe they will bring him back!