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 [1]matica:~$ dpkg --status slim
Package: slim
Status: unknown ok not-installed
Priority: optional
Section: x11
 [2]matica:~$ cat /etc/X11/default-display-manager
/usr/bin/slim
 [3]matica:~$ cat /etc/X11/default-display-manager.dpkg-tmp
/usr/bin/xdm
 [4]matica:~$

Control: tags -1 confirmed
on purge, when slim is the last one installed of the display manager
group [slim gdm sddm lightdm xdm wdm ..]
/etc/X11/default-display-manager file is left there with /usr/bin/slim
as content and also
/etc/systemd/system/display-manager.service is left dangling
(see also #776173)

and when there are other display manager installed in the system
/etc/X11/default-display-manager content is updated but
/etc/systemd/system/display-manager.service target is still
/lib/systemd/system/slim.service