/usr/share/debhelper/autoscripts/prerm-wm is in debhelper 9.20120115ubuntu3.
This file is owned by root:root, with mode 0o644.
The actual contents of the file can be viewed below.
1 2 3 | if [ "$1" = "remove" ]; then
update-alternatives --remove x-window-manager #WM#
fi
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