The eBook Reader blog details which Kindle generations lose over-the-air Send to Kindle and how Amazon is phasing older file pipelines alongside unsupported devices - compressing the "official" path for loading books onto aging hardware.
The practical upshot is sideloading via USB and manual file management: workable for tech-comfortable readers, annoying for everyone who liked email-to-Kindle simplicity. It's also a reminder that DRM-free backups matter when platforms sunset quietly.
(Shortened and summarised to avoid devaluing the source)