drws, the manpage suggests that -Rn doesn't additionally remove backups, but rather prevents .pacsave files from being created in the first place. Unless you've encountered a bug that actually does remove additional files, "pacman -Ql package_to_be_removed" provides a list of files for the package, pacman -Fl removed_package should provide a list of all the files after removal. Similarly, pacman -[QS]i can give you an idea of the total space gain/ed.

Did you have a use case in mind that isn't covered by those options?

You're right, -Rn only prevents creation of backup files. However, by doing that, it effectively prevents any package remainings to be left in the system (with an exception of package in the cache directory) and it is a closest pacman equivalent of Debian's "apt-get purge". The space "gained" by not creating the files is maybe really not the most necessary information to display here as pacman also doesn't output space-related information during other operations, but outputting a list of prevented backup files when using the -Rn options and similarly a list of backed-up files during a normal -R operation still seems a reasonable suggestion.

I'm going to close this as "Won't Implement". Using the -n option means you don't want those files backed up. Reporting on files you don't want is counterintuitive, and I do not want pacman's output being cluttered by this.