It seems to me that apart from some counted Moocs (where exercises are somewhat limited) a bank to study more abstract areas of mathematics, memorizing, solving problems and checking for correct proofs is lacking, the only thing that comes to mind is creating some supermemo cloze excercices to study theorems and important demonstration structures of the books of my courses, but i'm finding this lack of automation an opportunity to help more people get into higher math.
Even a page like https://www.canfigureit.com/ or the ancient https://feromax.com/cgi-bin/ProveIt.pl?task=getproofslist are of help for me, because of the very lack of rigorous background even on geometry, this may seem desperate but with the fear of having only two midterms and a final in most of the courses that will come from my department, and where all my previous knowledge helps only into my CS classes -and only the non rigorous ones-, finding this stack exchange feels like a little treasure, that I'll take it into account when I study Real analysis and Topology in the semesters to come, but i wonder for any other non human practice help...