At the end of season 2 Michael goes to the future, and some of the crew go with the Discovery to the future with her. Which is followed in Season 3. Others, like Pike and Spock, stay in the present/past/whatever, and eventually in Strange New Worlds. The whole existence of Michael and the Discovery gets classified, which retroactively explains both why Spock never mentioned having a sister, and why the Spore drive was never mentioned again, especially when a whole series could have been cut short with it.
But why did Spock stay? Why not go to the future with her and the crew that did?
But why did Spock stay? Why not go to the future with her and the crew that did?