When using an old Ubuntu release, the package URLs stored in /etc/apt/sources.list no longer point to valid addresses. Eventually this causes apt update to throw errors and notices.

To fix this, the sources URLs in sources.list should be updated. Here is a command to do that:

# backup your the file we will change
cp /etc/apt/sources.list /etc/apt/sources.list.bak

# update the URLs with the "archive" ones
sed -i -re 's/([a-z]{2}.)?archive.ubuntu.com|security.ubuntu.com/old-releases.ubuntu.com/g' /etc/apt/sources.list

# run update again to check
apt update

# if everything checks out ok, remove the backup file
rm -f /etc/apt/sources.list.bak

^{A Thank You to Dean Lewis, the author of the source: https://veeamvanguards.com/ubuntu-apt-get-update-fails-the-repository-no-longer-has-a-release-file/}