SUMMARY
With the machines I help administer, we have been finding situations where the /boot directory is filling-up beyond 3 kernels on LUKS encrypted systems. apt autoremove is not removing old kernels as expected, nor is unattended-upgrades on the Ubuntu machines picking-up on the removable kernels.

We have had a system with as many as 15 kernel packages installed as a result of this not working as expected.

The majority of these machines are using Discover to do their package upgrading, which uses PackageKit as its backend.

This has led us to believe that Discover and Packagekit are possibly marking apt packages that should be marked as "auto" as "manual". We were able to get unattended-upgrades to to the trick and "apt autoremove" to do its job by using "apt-mark auto linux-image-${version}-generic). However, this manual intervention should not be required.

Per this downstream bug, we have confirmed that the problem does not lie in apt nor unattended-upgrades.

STEPS TO REPRODUCE
1. Run Kubuntu normally on a LUKS encrypted system
2. Wait a few upgrades on the kernels over time
3. Kernels do not get automatically removed and /boot partition fills-up.

OBSERVED RESULT
Boot partition gets full blocking further upgrades of any packages. Unused/old kernel images do not get automatically removed. Investigation shows new kernel versions are marked as manually installed.

EXPECTED RESULT
Unused kernel images should be automatically removed upon reboot and/or using "apt autoremove". Applications installed from a metapackage (case in point, linux-image-generic) should be marked as auto-installed, allowing the autoremove process to work as expected.

SOFTWARE/OS VERSIONS
Linux/KDE Plasma: Ubuntu 20.04 LTS
(available in About System)
KDE Plasma Version: 5.18.5
KDE Frameworks Version: 5.68.0
Qt Version: 5.12.8

ADDITIONAL INFORMATION