System Info:  Dell 4600, LXLE 18.04.3 (i386)
Symptoms:  “apt  autoremove --purge” tries to remove all kernels, warning before removal of the running kernel.  Advertised behavior is to remove all kernels except running kernel and next most recent kernel, so 2 kernels should remain after doing autoremove.
Background: Executable file “/etc/kernel/postinst.d/apt-auto-removal" searches the apt configuration files in directory  /etc/apt/apt.conf.d  for a section labeled ‘APT::VersionedKernelPackages’ and uses this as input to generate the “/etc/apt/apt.conf.d/01autoremove-kernels” configuration file.  File ‘01autoremove-kernels’ contains the section  ‘’APT::NeverAutoRemove’ which is read by the ‘apt autoremove’ command to determine which kernels to keep.
Source of Problem:  LXLE 18.04.3 appears to be missing an apt configuration file containing the
section labeled ‘APT::VersionedKernelPackages’, resulting in  “/etc/kernel/postinst.d/apt-auto-removal" creating an “/etc/apt/apt.conf.d/01autoremove-kernels” config file that contains an empty section for ‘’APT::NeverAutoRemove’, even though all of the debug info is there.  Thus “apt autoremove” thinks there are no kernels to be retained.
Possible fix: Lubuntu 18.04.3, as installed, contains apt configuration file “/etc/apt/apt.conf.d/01autoremove” which contains the ‘APT::VersionedKernelPackages’ section that is needed by “/etc/kernel/postinst.d/apt-auto-removal”.

The above represents my best understanding of the situation, but should not be taken as gospel.  Hope I have not turned everyone’s mind to mush with this rendition........