/lib/systemd/system-shutdown/nutshutdown is in nut-server 2.7.4-5.1ubuntu2.
This file is owned by root:root, with mode 0o755.
The actual contents of the file can be viewed below.
1 2 | #!/bin/sh
/sbin/upsmon -K >/dev/null 2>&1 && /sbin/upsdrvctl shutdown
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