/etc/apt/apt.conf.d/99update-notifier is in update-notifier-common 0.154.1.
This file is owned by root:root, with mode 0o644.
The actual contents of the file can be viewed below.
1 | DPkg::Post-Invoke {"if [ -d /var/lib/update-notifier ]; then touch /var/lib/update-notifier/dpkg-run-stamp; fi; if [ -e /var/lib/update-notifier/updates-available ]; then echo > /var/lib/update-notifier/updates-available; fi "; };
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