Comment 6 for bug 1797209

If libc6 is not in reboot-required.pkgs you can upgrade to the next release.

bdmurray@clean-bionic-amd64:/tmp$ cat /run/reboot-required.pkgs
evolution-data-server
linux-base
bdmurray@clean-bionic-amd64:/tmp$ do-release-upgrade -d
Checking for a new Ubuntu release
Get:1 Upgrade tool signature [819 B]
Get:2 Upgrade tool [1,263 kB]
Fetched 1,264 kB in 0s (0 B/s)
authenticate 'cosmic.tar.gz' against 'cosmic.tar.gz.gpg'
extracting 'cosmic.tar.gz'

If libc6 is in it though you cannot upgrade to the next release.

bdmurray@clean-bionic-amd64:/tmp$ cat /run/reboot-required.pkgs
evolution-data-server
linux-base
libc6
bdmurray@clean-bionic-amd64:/tmp$ do-release-upgrade -d
Checking for a new Ubuntu release
You have not rebooted after updating a package which requires a reboot. Please reboot before upgrading.