/lib/partman/undo.d/70unbackup is in ubiquity 2.18.7.
This file is owned by root:root, with mode 0o755.
The actual contents of the file can be viewed below.
1 2 3 4 5 6 7 8 9 | #!/bin/sh
[ -d /var/lib/partman/backup ] || exit 0
if [ -d /var/lib/partman/devices ]; then
rm -rf /var/lib/partman/devices
fi
cp -a /var/lib/partman/backup /var/lib/partman/devices
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