Script, that commonly is used to it in %postun is empty and does nothing. It was wrote even in README:
/usr/share/doc/rpm-helper/README wrote:%_postun_userdel <name>
Does nothing currently.
...
%_postun_groupdel <group>
Does nothing currently.
What is methodologically (technically) correct - to remove or not to remove system user (with userdel -rf $NAME) with package uninstallation?