Comment 4 for bug 1951903

Running that with some options for verbosity confirmed that it isn't the deluser/delgroup calls that break.
Instead later on in the postrm (/var/lib/dpkg/info/libvirt-daemon-system.postrm) the generated snippets enter

# Automatically added by dh_installdebconf/11.1.6ubuntu2
if [ "$1" = purge ] && [ -e /usr/share/debconf/confmodule ]; then
        . /usr/share/debconf/confmodule
        db_purge
fi
# End automatically added section

That section triggers this log:

+ [ -e /usr/share/debconf/confmodule ]
+ . /usr/share/debconf/confmodule
+ [ ! 1 ]
+ [ -z ]
+ exec
+ [ ]
+ exec
+ DEBCONF_REDIR=1
+ export DEBCONF_REDIR
+ db_purge
+ _db_cmd PURGE
+ _db_internal_IFS=

+ IFS=
+ printf %s\n PURGE
+ IFS=

+ IFS=
 read -r _db_internal_line
+ RET=20 Unsupported command "removing" (full line was "Removing group `libvirtd' ...") received from confmodule.
+ return 20

So the issue seems about removing group libvirtd (and not libvirt-dnsmaq as it first seemed) and somehow that error message creeps into the debconf handling and then breaks it there.