Your desire for transparency is commendable, but it cannot lead to our team spending (more) hours (than we already do), to tell you everything in the world about our business. We do our best, and appreciate your tolerance of us needing to support millions of other customers as well.
As you posit, we weren’t trying to hide from anyone why drag and drop was misbehaving for some users lately.
For the most part, it has never worked well because folks try to upload massive sites that require too much memory in the browser for their machine, or their network is not stable, and those things cause the upload to hang. So, that’s why our team wrote this support guide 3 and a half years ago, one of the first ones we wrote: [Support Guide] My drag and drop deploy is stuck in "Uploading" status
This primary failure cause for this least-desirable and least-capable upload method has been the case in the past, and will be in the future too.
But, why did drag and drop uploads break more than usual for some people with some sites lately? A bug, which we fixed, which popped up while we made otherwise invisible-to-you improvements in our authentication in browser while people use our UI. It impacted mostly sites with larger than average files, so our tests did not catch that misbehavior.