The latest generation, Kindle Paperwhite, Kindle Scribe 2, Kindle Scribe 1, and Kindle and the upcoming Kindle Colorsoft, all have something in common. When you plug them into USB into your PC or MAC, they no longer appear as external drives. This prevents users from using file managers to back up their books or to sideload new books onto the Kindle. Amazon has also removed the download and transfer via USB option for purchased ebooks from the content page. This will likely prevent people from striping the DRM from the books and sharing them on piracy websites. This is not
Calibre has always been a small price to pay, but if sideloading goes away, I’ll certainly never “upgrade” again, and I’ll trash my 11th gen Paperwhite if they somehow make it stop working. Usable e-ink ereaders are even doable as DIY projects now, and Kobo will probably stay less closed-off than Amazon for a good while.
That said, reading the comments and the article it seems like as long as your OS (or some app) supports MTP, everything should still work more or less as it has, which is to say kind of annoying and with Amazon pulling little microaggressions like deleting your cover thumbnails, but overall sideloading should still function.