/sbin/coldreboot is in kexec-tools 1:2.0.2-3ubuntu4.
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 | #!/bin/sh
NOKEXECFILE=/tmp/no-kexec-reboot
/bin/rm -f $NOKEXECFILE
touch $NOKEXECFILE
/sbin/reboot $*
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