Usually an unsupported device stops getting new functionality and security fixes. The unsupported Kindles lose existing functionality, i.e. the ability to add books. Not quite bricked unlike, say, Sonos, but you are limited to the books y already downloaded to them.

This is inherent to DRM, and the reason why I would never have considered buying one in the first place. The eReader I have is a PocketBook Versa. Same price as a Kindle, extensible using microSD and I can add my non-DRM books however I want. Fortunately, Apple Books ePub FairPlay DRM is fairly easy to remove, so that's where I buy them.