Don't know, sorry. I did this the 19th of February according to my logs, via the "DeDRM Plugin (v10.0.3)" in Calibre and it worked fine then at least. Seems this is the repository for the plugin: https://github.com/noDRM/DeDRM_tools

For downloading the files themselves, I think I ended up using this: https://github.com/treetrum/amazon-kindle-bulk-downloader

Edit: Oh, just saw the warning from the bulk downloader's README now :/

> As of 26th February 2025, Amazon has removed the ability to download backups of your Kindle books. Unfortunately, this means that this tool is no longer functional.