Update for bookworm: In /etc/adduser.conf all settings are commented out with their default values. cf.adduser in debian-edu-config uses a simple edit_line search and replace which does not understand comments so it just replaces those commented out defaults. That means adduser will use the internal defaults and create local users with UID/GID 1000 and higher.
In addition to systemd, polkitd now also uses a UID above 499, on a main server with MATE desktop I have the following UIDs above 499: 995 polkitd 997 systemd-timesync 998 systemd-network -- Guido Berhoerster