Seems to a part of ubuntu-advantage-tools
to distinguish between freely available security updates (standard) and ESM (paid, later, actual for 16.04 LTS).
May be removed by removing special UA hook by
sudo rm /etc/apt/apt.conf.d/20apt-esm-hook.conf
Above will transform
42 upgraded, 5 newly installed, 0 to remove and 0 not upgraded.
29 standard security updates
to normal traditional behaviour
42 upgraded, 5 newly installed, 0 to remove and 0 not upgraded.
All this UA ESM stuff on supported Ubuntu versions smells like big BUY word with capital letters.
I found this unacceptable. Especially the viral behavior of UA:
$ sudo apt autopurge ubuntu-advantage-tools
Reading package lists... Done
Building dependency tree
Reading state information... Done
The following packages will be REMOVED:
distro-info* python3-debconf* ubuntu-advantage-tools* ubuntu-mate-core* ubuntu-mate-desktop* ubuntu-minimal* ubuntu-release-upgrader-core*
ubuntu-release-upgrader-gtk* update-manager* update-manager-core* update-notifier* update-notifier-common*
0 upgraded, 0 newly installed, 12 to remove and 41 not upgraded.
After this operation, 6 687 kB disk space will be freed.
Do you want to continue? [Y/n] n
Abort.
I have reported a bug 1950692 about ubuntu-advantage-tools
"integration".
Clues: