This is not generally the case: "apt only allows one version of something to installed at once" - apt and dpkg always supported having different major versions

This is not generally the case: "apt only allows one version of something to installed at once" - apt and dpkg always supported having different major versions of a shared library installed concurrently. See eg https://www.debian.org/doc//debian-policy/ch-sharedlibs.html https://www.debian.org/doc//debian-policy/ch-sharedlibs.html which says > The run-time shared library must be placed in a package whose name changes whenever the SONAME of the shared library changes. This allows several versions of the shared library to be installed at the same time, allowing installation of the new version of the shared library without immediately breaking binaries that depend on the old version. In this libunwind case the situation is more complicated, the its different versions have conflicting subdependencies.