systemd-rfkill records bluetooth state on every change, and restores it on boot. So it seems when set to remember, all bluedevil has to do on boot is do-nothing.
I think this is more robust than the current approach of saving on kded exit. There’re a few “bluetooth not restored” bug reports:
https://bugs.kde.org/show_bug.cgi?id=457130
https://bugs.kde.org/show_bug.cgi?id=445376
Of course, the current approach saves the state per-user, while systemd saves it globally. Even so, perhaps it would be more robust to save it on every state change, like systemd does?
Oh and I bring this up because I just figure out that the bluetooth is not restored on my laptop because kded never cleanly quits:
https://bugs.kde.org/show_bug.cgi?id=476677