There is no good path for non technical users to upgrade end of life versions of Ubuntu.
Explanation and suggestion below.
I am a 25 year user of linux and at least a decade on ubuntu. I also am an it professional so have little problem diving below the interface to fix stuff or do non standard configuration. However I prefer stuff to just work and not have to go below the hood so to say. More importantly is all those people that simply don't have the ability to go below the hood. I was running Zesty which was released just over a year ago and suddenly was unable to update or do a dist upgrade. I understand it was not a LTS release and am ok with updates not happening, but I was running it because 17.10 broke some of my software I use. However there is no upgrade path on a zesty computer at this time without manually changing your apt sources files. To me this is unacceptable.
My suggestion is to even if you disable updates or even installation of other software from the repositories for the outdated distro that you leave the ability to do a dist upgrade. It could be as simple as a option to dist upgrade that runs a scrip like I just ran to change to the archived version of the zesty repository and then rant the dist upgrade. I can also think of several other ways.
what I did was use
sudo sed -i -re 's/([a-z]{2}\.)?archive.ubuntu.com|security.ubuntu.com/old-releases.ubuntu.com/g' /etc/apt/sources.list
and then
sudo apt-get update && sudo apt-get dist-upgrade
For a technical user this is a relatively easy fix to the problem. For a non technical user that is a multi year educational journey to learn how to do this, they don't want this knowledge they just want their computer to work. I know that it should be a fairly easy fix for the people that maintain the gui upgrade.
My wife or mom or many other people I have using Ubuntu desktops wouldn't know how to do this. It is a major failure point as a desktop to have a version just over a year old that someone can't click a button to update to the next version. I would propose that it shouldn't matter how old it is that there should always be left a standard upgrade path to the next version even if you are no longer supporting it. This type of major disconnect in usability is what pushes normal computer users to just go back to windows or mac. Even I was really irritated that I had to figure out why updating and installing software quit working and then figure out how to fix it.
My overall suggestion would be not to archive older distro's just put in a pop up when you update or install software that your distro is no longer supported and click here to update. It would seem to be a fairly simple solution.
We do need to move the files off archive.ubuntu.com for space reasons on mirrors, but (1) that's not actually done by Launchpad as such, but by Ubuntu's end-of-life processes, and (b) this is the kind of thing that ought to be handled by the distribution's upgrade tools. I've heard a few developers talking recently about the need to improve this. Reassigning over to ubuntu-