I occasionally get a warning message, notifying me that my
boot volume is low on space.
I have not found a way of removing old kernels automatically, and I've yet to script it, so for now this is just a reminder for me of what to do.
Check which kernels are installed
dpkg --list | grep linux-image
Remove the kernels I no longer want
apt remove --purge [name of kernel from list generated using the command above]