Jeremy Renner was unable to reprise his role due to filming Avengers: Endgame. Renner recently revealed that he was asked to come back for a week of shooting and his character would be killed off. Which Renner refused. Believing the character deserved more of a send off.
This is the same Max played by Vanessa Redgrave that Ethan has dealings with in the very first Mission Impossible movie. This is the White Widow's mother.
A plot device from another Mission Impossible movie.
