This may have way more information that you seek... But it does cover the question you ask.
There is even a link to a video that explains the reasoning.
See: Production Chain Changes - API Announcements - Let's Encrypt Community Support (letsencrypt.org)
I don't really see a question in "a".
For "b": Running curl with -k is not safe at all. It would allow a MITM to use any cert at all and succeed.