APT Recommended Package Retention Across Release Upgrades

When APT transitions a system between Debian release branches, the handling of Recommended dependencies influences final package footprint and system consistency. The objective is to determine whether the dependency solver retains Recommended packages that are no longer required by newly installed software, or removes them based on updated dependency graphs, and how this decision interacts with active repository pinning configurations.

A central uncertainty concerns the solver's priority classification: Recommended packages historically carry more weight than Suggested ones during resolution, yet behavior may shift when release branches change, when pinning introduces packages from alternative repositories, or when conflict resolution demands removing previously satisfied dependencies. The degree to which metadata versioning across branches affects the solver's conservative versus aggressive retention strategies remains an open design question.

Does the solver treat Recommended dependencies as persistent constraints unless explicitly removed, or does it reclassify them as soft dependencies once the source package's version changes? How does active pinning of a package to a specific release branch affect the solver's choice to retain or purge Recommended counterparts? In a mixed-repository setup where stable and testing sources coexist, does the presence of a Recommended package from the target branch override the current branch's dependency rules during a partial upgrade?