/etc/apt/apt.conf.d/99update-notifier is in update-notifier-common 3.192.
This file is owned by root:root, with mode 0o644.
The actual contents of the file can be viewed below.
1 2 | DPkg::Post-Invoke {"if [ -d /var/lib/update-notifier ]; then touch /var/lib/update-notifier/dpkg-run-stamp; fi; /usr/lib/update-notifier/update-motd-updates-available 2>/dev/null || true";};
APT::Update::Post-Invoke-Success {"/usr/lib/update-notifier/update-motd-updates-available 2>/dev/null || true";};
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