/etc/acpi/events/thinkpad-mute is in acpi-support 0.142-8.
This file is owned by root:root, with mode 0o644.
The actual contents of the file can be viewed below.
1 2 3 | # Volume Always Mute
event=ibm/hotkey HKEY 00000080 00001017
action=/etc/acpi/always-mute.sh
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