Tested with 1.1ubuntu1.18.04.7~16.04.2 on Xenial:

root@x-uu-lp-1260041:~# apt-mark auto linux-image-extra-4.8.0-56-generic linux-image-extra-4.8.0-58-generic linux-image-extra-4.8.0-54-generic linux-image-extra-4.8.0-53-generic
linux-image-extra-4.8.0-56-generic set to automatically installed.
linux-image-extra-4.8.0-58-generic set to automatically installed.
linux-image-extra-4.8.0-54-generic set to automatically installed.
linux-image-extra-4.8.0-53-generic set to automatically installed.
root@x-uu-lp-1260041:~# unattended-upgrade --verbose
Initial blacklisted packages: 
Initial whitelisted packages: 
Starting unattended upgrades script
Allowed origins are: o=Ubuntu,a=xenial, o=Ubuntu,a=xenial-security, o=UbuntuESM,a=xenial, o=Ubuntu,a=xenial-updates
Removing unused kernel packages: linux-image-extra-4.8.0-54-generic linux-image-4.8.0-54-generic
Keeping auto-removable linux-image-extra-4.8.0-54-generic package(s) because it would also remove the following packages which should be kept in this step: libpam-systemd libsystemd0 libudev1 systemd systemd-sysv udev
(Reading database ... 53554 files and directories currently installed.)
Removing linux-image-extra-4.8.0-54-generic (4.8.0-54.57~16.04.1) ...
run-parts: executing /etc/kernel/postinst.d/apt-auto-removal 4.8.0-54-generic /boot/vmlinuz-4.8.0-54-generic
run-parts: executing /etc/kernel/postinst.d/initramfs-tools 4.8.0-54-generic /boot/vmlinuz-4.8.0-54-generic
update-initramfs: Generating /boot/initrd.img-4.8.0-54-generic
W: mdadm: /etc/mdadm/mdadm.conf defines no arrays.
run-parts: executing /etc/kernel/postinst.d/unattended-upgrades 4.8.0-54-generic /boot/vmlinuz-4.8.0-54-generic
run-parts: executing /etc/kernel/postinst.d/update-notifier 4.8.0-54-generic /boot/vmlinuz-4.8.0-54-generic
run-parts: executing /etc/kernel/postinst.d/zz-update-grub 4.8.0-54-generic /boot/vmlinuz-4.8.0-54-generic
Removing linux-image-4.8.0-54-generic (4.8.0-54.57~16.04.1) ...
Examining /etc/kernel/postrm.d .
run-parts: executing /etc/kernel/postrm.d/initramfs-tools 4.8.0-54-generic /boot/vmlinuz-4.8.0-54-generic
update-initramfs: Deleting /boot/initrd.img-4.8.0-54-generic
run-parts: executing /etc/kernel/postrm.d/zz-update-grub 4.8.0-54-generic /boot/vmlinuz-4.8.0-54-generic
Packages that were successfully auto-removed: linux-image-4.8.0-54-generic linux-image-extra-4.8.0-54-generic
Packages that are kept back: linux-image-extra-4.8.0-54-generic
Packages that will be upgraded: libpam-systemd libsystemd0 libudev1 systemd systemd-sysv udev
Writing dpkg log to /var/log/unattended-upgrades/unattended-upgrades-dpkg.log
(Reading database ... 47681 files and directories currently installed.)
Preparing to unpack .../systemd-sysv_229-4ubuntu21.17_amd64.deb ...
Unpacking systemd-sysv (229-4ubuntu21.17) over (229-4ubuntu21.16) ...
Processing triggers for man-db (2.7.5-1) ...
Setting up systemd-sysv (229-4ubuntu21.17) ...
Log ended: 2019-03-13  17:17:36

Log started: 2019-03-13  17:17:36
(Reading database ... 47681 files and directories currently installed.)
Preparing to unpack .../udev_229-4ubuntu21.17_amd64.deb ...
Unpacking udev (229-4ubuntu21.17) over (229-4ubuntu21.16) ...
Preparing to unpack .../libudev1_229-4ubuntu21.17_amd64.deb ...
Unpacking libudev1:amd64 (229-4ubuntu21.17) over (229-4ubuntu21.16) ...
Processing triggers for man-db (2.7.5-1) ...
Processing triggers for systemd (229-4ubuntu21.16) ...
Processing triggers for ureadahead (0.100.0-19) ...
Processing triggers for libc-bin (2.23-0ubuntu11) ...
Setting up libudev1:amd64 (229-4ubuntu21.17) ...
Setting up udev (229-4ubuntu21.17) ...
addgroup: The group `input' already exists as a system group. Exiting.
update-initramfs: deferring update (trigger activated)
Processing triggers for libc-bin (2.23-0ubuntu11) ...
Processing triggers for initramfs-tools (0.122ubuntu8.14) ...
update-initramfs: Generating /boot/initrd.img-4.15.0-45-generic
W: mdadm: /etc/mdadm/mdadm.conf defines no arrays.
Log ended: 2019-03-13  17:17:44

Log started: 2019-03-13  17:17:45
(Reading database ... 47681 files and directories currently installed.)
Preparing to unpack .../libpam-systemd_229-4ubuntu21.17_amd64.deb ...
Unpacking libpam-systemd:amd64 (229-4ubuntu21.17) over (229-4ubuntu21.16) ...
Preparing to unpack .../libsystemd0_229-4ubuntu21.17_amd64.deb ...
Unpacking libsystemd0:amd64 (229-4ubuntu21.17) over (229-4ubuntu21.16) ...
Processing triggers for man-db (2.7.5-1) ...
Processing triggers for libc-bin (2.23-0ubuntu11) ...
Setting up libsystemd0:amd64 (229-4ubuntu21.17) ...
Processing triggers for libc-bin (2.23-0ubuntu11) ...
(Reading database ... 47681 files and directories currently installed.)
Preparing to unpack .../systemd_229-4ubuntu21.17_amd64.deb ...
Unpacking systemd (229-4ubuntu21.17) over (229-4ubuntu21.16) ...
Processing triggers for dbus (1.10.6-1ubuntu3.3) ...
Processing triggers for man-db (2.7.5-1) ...
Processing triggers for ureadahead (0.100.0-19) ...
Setting up systemd (229-4ubuntu21.17) ...
addgroup: The group `systemd-journal' already exists as a system group. Exiting.
[/usr/lib/tmpfiles.d/var.conf:14] Duplicate line for path "/var/log", ignoring.
Setting up libpam-systemd:amd64 (229-4ubuntu21.17) ...
All upgrades installed