It no longer does for the first few generations of Kindles. My Kindle Keyboard (16 years old, but it still works and I still use it almost daily) is essentially a useless paperweight now, as far as Amazon is concerned. De-registering it will brick it and they shut down the "send file to e-mail" service, too. Can no longer buy and download books off Amazon either (they shut that down in 2024, afaik).
If not for Calibre, I would have absolutely no option of ever getting new books on it.
If not for Calibre, I would have absolutely no option of ever getting new books on it.