/usr/share/zsh/functions/Misc/harden is in zsh-common 5.0.2-3ubuntu6.
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 | #!/bin/sh
# harden a link (convert it to a singly linked file)
cp $1 $1.foo
rm $1
mv $1.foo $1
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