You can use MOTD (message of the day) to let you know if a Debian server requires reboot and why upon login.
Create a new file named /etc/update-motd.d/98-reboot-required
and add to it the following lines:
#!/bin/sh -e
#
# helper for update-motd
if [ -f /var/run/reboot-required ]; then
echo "*** System restart required ***"
cat /var/run/reboot-required.pkgs
fi
Make the file executable:
$ sudo chmod +x /etc/update-motd.d/98-reboot-required
Now, you can test the new MOTD script using:
$ run-parts --lsbsysinit /etc/update-motd.d
If you have any installed updates that require reboot, you will get a message stating so, with a list of the packages that require the reboot.
*** System restart required ***
linux-image-5.10.0-19-cloud-amd64