Spider Mapping

"in June of 2020, Khan Academy’s product team made the decision to deprecate Missions and, because the World of Math Course was directly tied to that feature, it has been deprecated as well."

It seems that over time, they have started reintroducing a similar feature. You now have mastery learning in courses, which also provide a comparable 'World of Math' experience, reminiscent of what was available back in the day. However, the original feature is indeed gone.