What is Ubuntu Advantage doing on my (fully supported) 20.04 box, what is it checking for twice a day, and can I get rid of it?

This fine answer works great for 18.04.

It tells you to remove the package which is great because on 18.04 it only removes the additional package ubuntu-minimal.

However, on 20.04 it wants to remove a lot more stuff, namely

• ttf-mscorefonts-installer
• ubuntu-release-upgrader-gtk
• update-manager
• update-manager-core
• update-notifier
• update-notifier-common

because of the following dependency tree:

$ aptitude why ubuntu-advantage-tools
i   ttf-mscorefonts-installer Depends update-notifier-common (>= 0.119ubuntu2)
i A update-notifier-common    Depends update-manager-core (>= 1:17.04.2)      
i A update-manager-core       Depends ubuntu-advantage-tools           

That's unacceptable, so we have to look for another way. And here it is.

1. Stop and mask the services

   sudo systemctl stop ua-messaging.timer
   sudo systemctl stop ua-messaging.service
   sudo systemctl stop ua-timer.timer
   sudo systemctl mask ua-messaging.timer
   sudo systemctl mask ua-messaging.service
   sudo systemctl mask ua-timer.timer
   

2. Check that it worked

   systemctl status ua-messaging.service
   systemctl status ua-messaging.timer
   systemctl status ua-timer.timer
   

   You should see a status like this for each

   $ systemctl status ua-messaging.service
   * ua-messaging.service
       Loaded: masked (Reason: Unit ua-messaging.service is masked.)
       Active: inactive (dead)
   $ systemctl status ua-messaging.timer
   * ua-messaging.timer
       Loaded: masked (Reason: Unit ua-messaging.timer is masked.)
       Active: inactive (dead) since Fri 2021-11-12 23:10:16 GMT; 1min 2s ago
      Trigger: n/a
   
   Nov 12 01:09:42 focal systemd[1]: Started Ubuntu Advantage update messaging.
   Nov 12 01:10:16 focal systemd[1]: ua-messaging.timer: Succeeded.
   Nov 12 01:10:16 focal systemd[1]: Stopped Ubuntu Advantage update messaging.
   $ systemctl status ua-timer.timer
   * ua-timer.timer
       Loaded: masked (Reason: Unit ua-timer.timer is masked.)
       Active: inactive (dead)
      Trigger: n/a
   

3. Remove the APT hook (see this answer for details why)

   sudo rm /etc/apt/apt.conf.d/20apt-esm-hook.conf
   

and after a day or so check /var/log/ubuntu-advantage.log to confirm it is not running any more.