This seems to be part of another cycle of Amazon locking down access to Kindle ebooks.
I saw indications that Amazon was planning to block book downloads to older Kindle devices
almost a year ago. I expect this change to be announced worldwide soon.
It has come to light recently that the Kindle models that will still be supported can have their DRM updated without an actual firmware release. I believe that will now become more widespread.
And today I came across something else that hints of Amazon phasing out the Kindle for PC app, which has long been used to work around Kindle DRM.