Many major Linux distributions (like Red Hat Enterprise Linux, Ubuntu, Debian) freeze the packaged software versions, when a new major release of the distribution is created. For example Debian 10 packages the popular Apache webserver in version 2.4.38, while the current release (31.03.2020) is version 2.4.41. Bugfixes, and therefore also security fixes, must be backported to the packaged version of the Apache webserver (and all the other packaged software in the distribution). However, feature updates in newer software versions are not backported. This is done to guarantee a stable system with minimal changes.
TODO: elaborate more on the issue
On this site, I will try to collect examplary issues, when backporting failed.