I already posted this on "Spanish," but I am posting again because I think people should be able to consult Lesson Review in any language Duolingo offers in order to understand their mistakes. I have been consulting Lesson Review for months now in Spanish and it has been immensely helpful. I would really like for it to be put back.
I read somewhere that it's one of those things that got lost in the rewrite of the website because it wasn't ready yet, but is coming back, it's just not done yet. I don't have a reference for that at the moment though.
Edit: Found it: https://www.duolingo.com/comment/22894998