I think they've all evolved now to where that is only available for paid tiers.
One innovation I really enjoyed was marking and commenting another student's answers, once you'd submitted your own. There are flaws but it's a valuable variation on graded feedback.
I definitely agree with you on this. However, there are three things self-learners (or even students) can do.
1. Find a textbook with worked solutions or a different presentation of material.
2. Look for course material that has problem sets with solutions, worked tutorials, or worked examples.
3. Learn numerical or other methods to evaluate your solutions. I know this isn’t possible for proofs, but if it’s an algorithm’s run time, I don’t see how implementing the algorithm and plotting run times over different input sizes wouldn’t be a good way to see if you have the correct run time.
For me it's ok, I guess. I have graduated years ago and it's strictly for fun. But I wonder if it also creates a kind of artificial divide for those not able to attend a university but could have gotten good use of this kind of material by self study.
Edit, found this quote from him on an earlier discussion which summarizes it nicely:
> I'm honestly seriously torn about this. There is a serious tension between pedagogical needs of students in formal classrooms and the pedagogical needs of self-learners. I've chosen to aim for the former. Yes, I know it's a bummer.