I think the real problem here isn't http vs. https, but just that there's no visibility to the Cloudflare blocked ip list. So when someone has trouble accessing the Let's Encrypt API, it's hard to know if it's a Cloudflare-level block or something on the client network's end of things, and the only way to check is to bug an lestaff member (which community members try to avoid if they don't need to). If there were some public interface to check if an IP is on block list (maybe some web page with a Captcha-type thing), or something along those lines, it might be helpful. I think the initial suggestion was assuming that it was an easier way for Let's Encrypt to provide a way that would help test some of the possibilities, but perhaps there's a better approach.
The initial "announcement" of adding blocking at the CDN layer wasn't even posted in the "API Announcements" section because the information was going to be "stale fairly soon", and that was a year and a half ago… Hopefully a better approach is already on the to-do list somewhere? (Though I know that to-do lists tend to get longer rather than shorter.)