autoremove is used to remove packages that were automatically installed to satisfy dependencies for other packages and are now no longer needed as dependencies changed or the package(s) needing them were removed in the meantime.
All unused Linux kernel takes disk space on
/boot, hence you should delete them from the system.
In your case, you upgraded your kernel from version 5.4 to a newer version. If you manually install 2 chosen kernels, i.e., the first and the current one, then
apt-autoremove will only ever remove the older versions you didn't manually install, so you will always have those 2 options plus whatever the latest one is.
Listing all installed Linux kernel
Firstly, type the following dpkg command along with egrep command to display a list of all installed kernels.
dpkg --list | egrep -i --color 'linux-image|linux-headers'
Only list installed kernels
dpkg --list | grep -i -E --color 'linux-image|linux-kernel' | grep '^ii'
You can also check them manually by navigating to your
Check the amount of disk space available on the file system by typing the following df command:
Removing unused Kernels
Remove unused kernel
sudo apt autoremove
Remove unused kernel and residual files and configurations
sudo apt --purge autoremove