Support here is pretty loose. These devices were already not supported in the traditional sense. They were not getting firmware updates, they were just allowed to continue using Amazon's DRM scheme and connect to the store.

AFAIK it's still possible to authorize ancient supported ePub readers with Adobe Digital Editions and load up DRMed books from providers like Google Play even with devices like the Sony PRS-505 (e.g,https://www.sony.com/electronics/support/reader-digital-book...), despite them exiting the market over a decade ago. Kobo also has continued providing firmware updates to devices from 2011, and even their unsupported devices can still load books via ADE or the Kobo Desktop App.