No Kindle Unlimited, though, and library apps will probably drop support for the older stuff in time.

Jailbreak on very old Kindles is reasonably straightforward and the fact that Amazon hasn't even put out point releases to stop it (as the do with newer models) is a strong hint that they've just given up on maintaining them. I still have a K3 (Kindle Keyboard) that not only is jailbroken: it runs Tailscale.

Unprotected books, no problem. Anna's Archive + Calibre will keep working just fine.