I've seen posts from developers about why they will not be implementing the option to restart a challenge. https://support.khanacademy.org/hc/en-us/community/posts/115001114611/comments/360008519352
It seems that this decision is to foster a growth mindset.
This is my question: do all the developers (or at least the developers who get to make decisions) work in ideal conditions, or if they don't, have partners, assistants, or other support people who magically make any distractions go away?
My case was having a small child start pestering me for attention when I was doing the first couple of questions, but I'm sure there are plenty of other such situations. I answered one incorrectly because I couldn't focus, and immediately realised that I was not in an appropriate environment to work in, and wanted to 'cancel' what was an honest mistake, not a lack of understanding. Most decent apps allow for a 'cancel' or 'undo' option. For you to decide for me that my goal is 'growth mindset' is patronising. Anki, for example, will let you decide for yourself if you made a typo or not.
I cannot be the only mother caring for children in this situation, and other marginalised groups may be affected by this assumption as well. It's a horrible attitude to have.