I have been using missions for a few years now to brush up on skills that I know and find out what I don't know. I'll miss them if they are gone because I dislike the sluggish interface you normally have to use. In the Math 3 page, it says Looking for missions? The Mathematic III mission will be available until June 2020.
EDIT: Just saw the thing on the mission page. I'm sad to see them go and I hope the team considers bringing them back.