Are old package versions removed from their Ubuntu apt package repositories just because they're old?

Solution 1:

All latest packages for all releases are still available at the relevant URLs.

As an example we can select Midnight Commander:

• publishing history maybe obtained at https://launchpad.net/ubuntu/+source/mc
• its actual versions are found via search on packages.ubuntu.com, the files are available at http://archive.ubuntu.com/ubuntu/pool/universe/m/mc/ since 2013. You should not download them manually because of dependency hell, but they are located here.
• its obsolete versions are still located at https://old-releases.ubuntu.com/ubuntu/pool/universe/m/mc/ since 2004. You should not download them manually because of dependency hell, but they are located here.

If you have specific needs to get old (or exactly specified) package version with its URL - you can use my special Docker based script. More details may be found at this Q&A.