mark adduser as Protected:yes
apt dropped its dependency on adduser in October 2022 but eight source packages still require adduser in their maintainer script for a purge removal. To prevent packages from becoming unpurgable in Trixie, mark adduser as Protected:yes. This will prevent adduser from accidentally getting removed. In Trixie+1, this field can be removed.
See https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=1035654