Upgraded to ubuntu 18.04.5 LTS "Enable UA Infra" notification

When it noticed that you were out of support, it created some files in
/var/lib/ubuntu-advantage/messages

I'm not sure that there's anything which removes these files once they are no longer relevant. But you can remove them yourself: sudo rm /var/lib/ubuntu-advantage/messages/*

I think that you can also remove /etc/apt/sources.list.d/ubuntu-esm-infra.list to prevent this problem from resurfacing, I believe until the next time you let the system go out of support. It's possible that removing the package ubuntu-advantage-tools is desirable.

17 May 2021

Unfortunately, ubuntu-advantage-tools depends on ubuntu-minimal. What this means is ubuntu considers this adware to be essential to all ubuntu systems. You can not remove it without removing ubuntu-minimal, which would require also removing packages that are genuinely essential.

Ubuntu just keeps getting worse. In my case, the hardware clock is flaky. I therefore get these messages because the system thinks it is 2030 or something until the network time synchronization completes. It is annoying, and simply deleting the messages won't help.

Removing /etc/apt/sources.list.d/ubuntu-esm-infra.list may work. I'll give it a shot.