Comment 28 for bug 1678187

I managed to get some free space back on /boot.
I'm posting this in the hopes that what I did may inform any fixing process.
1) List kernels with `dpkg -l | tail -n +6 | grep -E 'linux-image-[0-9]+'`
2) Delete the oldest initrd with update-initramfs -d -k 5.8.0-59-generic. There was now 111Mb free on /boot
3) Run `linux-purge -f`. This did nothing, but it output (presumably from running apt) a "The following packages were automatically installed and are no longer required" message
4) Run `apt autoremove`. This removed some kernels and initrd files.

Now I have 256Mb free.

This would be easier if /boot was a little larger. The install process gives you the choice of "totally automatic" and "you're on your own, buddy!" when partitioning. One neat trick the old Centos install has was to allow you to pre-load the manual partitioning scheme with the default, which you could then modify.