The Verge has the story:
Starting on February 26th, 2025, Amazon is removing a feature from its website allowing you to download purchased books to a computer and then copy them manually to a Kindle over USB.
There are a few reasons why some Kindle users might miss this feature. It’s useful if you don’t have access to a Wi-Fi network, and although it’s a tedious process since purchased books can only be downloaded one at a time, there’s some peace of mind in knowing you have offline copies of all your books.
Personally, it doesn’t really impact me since I’ve fallen out of love with my Kindle. I find reading on it is a soulless experience. But for others, this may be important.The feature is also the easiest way to convert books purchased from Amazon to other formats like EPUB that can be used on alternative devices such as a Kobo. Books downloaded through Amazon’s website are delivered in the older AZW3 format which allows DRM to be easily removed using various software tools.