MA has that too. (Kind of, not exactly the same) For each course you can see a graph similar to https://mathacademy.com/img/screenshots/knowledge-graph-calc... with your current progress. The initial test (seriously comprehensive) gives you a summary of everything, so you can figure out the missing areas that way too.

FYI, the "why not just learn from a Khan Academy / MIT OpenCourseWare / a textbook / etc" question comes up from time to time, so I wrote up a detailed answer here: https://justinmath.com/why-not-just-learn-from-a-textbook/

TLDR: I learned from those kinds of resources myself, and while I came a long way, for the amount of effort I put into learning, I could have gone a lot further if my time were used more efficiently. That's the problem that Math Academy solves.

This was such a great overview of the whole 'math field'! I remember slowly going through it a while ago and trying to complete It was a huge task, but seeing what is there made the progress easy to measure and visible.

Fast forward some time in the future, and they have removed it...

I don't quite understand what you're trying to express. The function of Khan Academy was taken down several years ago. However, some traces can still be found online.

What do you mean about the function being taken down?