btw: The reason for the file not being cleaned up is that a wrong
version is used in the maintainer scripts:
dpkg-maintscript-helper rm_conffile /etc/needrestart/hook.d/30-pacman 3.5-4 --
"$@"
But according to changelog.Debian, it was removed in 3.5-5.
dpkg-maintscript-helper(1) says:
prior-version
Defines the latest version of the package whose upgrade should
trigger the operation.
...
For example, for a conffile removed in version 2.0-1 of a package,
prior-version should be set to 2.0-1~. This will cause the conffile
to be removed even if the user rebuilt the previous version 1.0-1
as 1.0-1local1. Or a package switching a path from a symlink
(shipped in version 1.0-1) to a directory (shipped in version
2.0-1), but only performing the actual switch in the maintainer
scripts in version 3.0-1, should set prior-version to 3.0-1~.
So in your case, this was never executed anywhere.
It needs to be aligned to the future version of the package, where this
is fixed, instead.
