I know that I should upgrade to a newer supported Ubuntu version if possible, but:
if you have no possibility to upgrade (for reasons not important here)
you have to use the old-releases archive from http://old-releases.ubuntu.com to still be able to install some software.
Are those archives not touched at all? Or does Ubuntu sometimes still update them to solve fatal security issues with critical vulnerabilities like the "Heartbleed Bug" in the old releases?
Or is there an alternative repository, I could use in my
sources.list on such an old system?
Or is there some list of fixes/backports, I could/should install on those old systems?