Please support Offline System Updates

[rbalint]

Unattended-upgrades could prepare packages to be installed using systemd's offline-updates mechanism after reboot as an alternative to installing updates during shutdown.
https://www.freedesktop.org/wiki/Software/systemd/SystemUpdates/

The basic idea is collecting the packages during shutdown that can be installed as offline-updates, preparing them to be installed on reboot, then installing them with u-u in minimal steps when the system is booted into offline-updates mode.
U-u could also be configured to always reboot after packages are installed in offline-updates mode to ensure the the reboot takes place even when packages miss asking for reboot.

[rbalint]

This feature is also tracked at: https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=741356

[a-detiste]

I would push for this at my workplace for the next platform call for tender starting in 2021;
if it were in bullseye first it would be perfect for peer reviews.