It is really strange to have ESM enabled on the 18.04 LTS which is supported.
Currently I have:
```
$ dpkg -S esm | grep ubuntu-advantage-tools
ubuntu-advantage-tools: /usr/lib/ubuntu-advantage/apt-esm-hook
ubuntu-advantage-tools: /usr/share/keyrings/ubuntu-advantage-esm-apps.gpg
ubuntu-advantage-tools: /etc/update-motd.d/91-contract-ua-esm-status
ubuntu-advantage-tools: /usr/lib/ubuntu-advantage/apt-esm-json-hook
ubuntu-advantage-tools: /usr/lib/python3/dist-packages/uaclient/entitlements/esm.py
ubuntu-advantage-tools: /etc/apt/apt.conf.d/20apt-esm-hook.conf
ubuntu-advantage-tools: /usr/share/keyrings/ubuntu-advantage-esm-infra-trusty.gpg
ubuntu-advantage-tools: /etc/update-motd.d/88-esm-announce
$ sudo grep -r esm /etc/
/etc/update-motd.d/88-esm-announce:stamp="/var/lib/ubuntu-advantage/messages/motd-esm-announce"
/etc/update-motd.d/91-contract-ua-esm-status:stamp="/var/lib/ubuntu-advantage/messages/motd-esm-service-status"
/etc/ubuntu-advantage/help_data.yaml:esm-apps:
/etc/ubuntu-advantage/help_data.yaml: its end of life. You can find out more about the esm service at
/etc/ubuntu-advantage/help_data.yaml: https://ubuntu.com/security/esm
/etc/ubuntu-advantage/help_data.yaml:esm-infra:
/etc/ubuntu-advantage/help_data.yaml: esm-infra provides access to a private ppa which includes available high
/etc/ubuntu-advantage/help_data.yaml: You can find our more about the esm service at
/etc/ubuntu-advantage/help_data.yaml: https://ubuntu.com/security/esm
/etc/apt/sources.list.d/ubuntu-esm-infra.list.distUpgrade:deb https://esm.ubuntu.com/infra/ubuntu xenial-infra-security main
/etc/apt/sources.list.d/ubuntu-esm-infra.list.distUpgrade:# deb-src https://esm.ubuntu.com/infra/ubuntu xenial-infra-security main
/etc/apt/sources.list.d/ubuntu-esm-infra.list.distUpgrade:deb https://esm.ubuntu.com/infra/ubuntu xenial-infra-updates main
/etc/apt/sources.list.d/ubuntu-esm-infra.list.distUpgrade:# deb-src https://esm.ubuntu.com/infra/ubuntu xenial-infra-updates main
/etc/apt/sources.list.d/ubuntu-esm-infra.list:deb https://esm.ubuntu.com/infra/ubuntu bionic-infra-security main
/etc/apt/sources.list.d/ubuntu-esm-infra.list:# deb-src https://esm.ubuntu.com/infra/ubuntu xenial-infra-security main
/etc/apt/sources.list.d/ubuntu-esm-infra.list:deb https://esm.ubuntu.com/infra/ubuntu bionic-infra-updates main
/etc/apt/sources.list.d/ubuntu-esm-infra.list:# deb-src https://esm.ubuntu.com/infra/ubuntu xenial-infra-updates main
/etc/apt/apt.conf.d/20apt-esm-hook.conf: "[ ! -f /usr/lib/ubuntu-advantage/apt-esm-hook ] || /usr/lib/ubuntu-advantage/apt-esm-hook post-invoke-stats || true";
/etc/apt/apt.conf.d/20apt-esm-hook.conf: "[ ! -f /usr/lib/ubuntu-advantage/apt-esm-hook ] || /usr/lib/ubuntu-advantage/apt-esm-hook post-invoke-success || true";
/etc/apt/apt.conf.d/20apt-esm-hook.conf: "[ ! -f /usr/lib/ubuntu-advantage/apt-esm-hook ] || /usr/lib/ubuntu-advantage/apt-esm-hook pre-invoke || true";
/etc/apt/apt.conf.d/20apt-esm-hook.conf: "[ ! -f /usr/lib/ubuntu-advantage/apt-esm-json-hook ] || /usr/lib/ubuntu-advantage/apt-esm-json-hook || true";
Binary file /etc/apt/trusted.gpg.d/ubuntu-advantage-esm-infra-trusty.gpg matches
```
Having `https://esm.ubuntu.com/infra/ubuntu bionic-infra-updates` in sources.list is strange.
ProblemType: Bug
DistroRelease: Ubuntu 18.04
Package: ubuntu-advantage-tools 27.0.2~18.04.1
ProcVersionSignature: Ubuntu 4.15.0-143.147-generic 4.15.18
Uname: Linux 4.15.0-143-generic x86_64
ApportVersion: 2.20.9-0ubuntu7.23
Architecture: amd64
CurrentDesktop: MATE
Date: Sat May 15 10:59:54 2021
InstallationDate: Installed on 2018-08-07 (1011 days ago)
InstallationMedia: Ubuntu-MATE 16.04.3 LTS "Xenial Xerus" - Release amd64 (20170801)
SourcePackage: ubuntu-advantage-tools
UpgradeStatus: Upgraded to bionic on 2021-05-15 (0 days ago)
The same happens on Ubuntu Desktop 16.04 LTS → 18.04 LTS upgrade.
So using flavor is not an issue.