Let's say you're using Ubuntu 13.04 (Raring Ringtail, released in April 2013) and it just went End-of-Life on you, because it's supported for only 6 months, and the deprecated packages are taken down after 12 months.
You'll probably figure this out the hard way. When you run
sudo apt-get update, it will eventually report these errors:
Ign http://archive.ubuntu.com raring-updates/universe Sources/DiffIndex Err http://security.ubuntu.com raring-security/main Sources 404 Not Found [IP: 18.104.22.168 80] Err http://security.ubuntu.com raring-security/universe Sources 404 Not Found [IP: 22.214.171.124 80] Err http://security.ubuntu.com raring-security/main amd64 Packages 404 Not Found [IP: 126.96.36.199 80] Ign http://archive.ubuntu.com raring-updates/main amd64 Packages/DiffIndex Err http://security.ubuntu.com raring-security/universe amd64 Packages 404 Not Found [IP: 188.8.131.52 80] W: Failed to fetch http://security.ubuntu.com/ubuntu/dists/raring-security/main/source/Sources 404 Not Found [IP: 184.108.40.206 80] W: Failed to fetch http://security.ubuntu.com/ubuntu/dists/raring-security/universe/source/Sources 404 Not Found [IP: 220.127.116.11 80] W: Failed to fetch http://security.ubuntu.com/ubuntu/dists/raring-security/main/binary-amd64/Packages 404 Not Found [IP: 18.104.22.168 80] W: Failed to fetch http://security.ubuntu.com/ubuntu/dists/raring-security/universe/binary-amd64/Packages 404 Not Found [IP: 22.214.171.124 80]
Visiting http://archive.ubuntu.com/ubuntu/dists does confirm that raring packages are indeed gone, while lucid, precise, saucy, and trusty are still there, as per their promised support schedule.
Some googling reveals that they're not deleted, but simply moved to http://old-releases.ubuntu.com/ubuntu/dists/, sort of a graveyard for insecure and unsupported packages.
apt-get update work, we simply need to edit
/etc/apt/sources.list and update all URLs from
archive.ubuntu.com to point to
old-releases.ubuntu.com, as per the following sed command:
sudo sed -i.bak -r 's/(archive|security).ubuntu.com/old-releases.ubuntu.com/g' /etc/apt/sources.list
sudo apt-get update seems to run without a hitch. Of course if you do this, you're accepting to use an insecure system, and should do
sudo apt-get install update-manager-core; do-release-upgrade as soon as possible.