apt makes a distinction between “removed” and “purged.” In both, the packages are uninstalled; in the former state, config files remain, and in the latter, those are also removed. Actually, that’s not quite the whole story.
A package can have no configuration files, yet still be in ”residual config” state when removed. This happens if a package defines a postrm maintainer script. These can have basically any shell commands in them, so their actions aren’t visible in any list-of-files.
The specific package I was looking into was a library, with a postrm script that ran ldconfig… during removal. The package was being shown in residual-config state because it had a script. Although that script would do nothing during purge, apt (and dpkg) can’t know that.
How to list residual-config packages: apt list 2>/dev/null | grep residual-config or dpkg -l | grep ^rc.
Listing configuration files: try one of these answers as this gets real complex, real fast.
Reading a postrm script: look at /var/lib/dpkg/info/{PACKAGE}[:{ARCH}].postrm (the ARCH component may not be present.)
