/etc/apt/apt.conf.d/15update-stamp is in update-notifier-common 0.119ubuntu8.1.
This file is owned by root:root, with mode 0o644.
The actual contents of the file can be viewed below.
1 | APT::Update::Post-Invoke-Success {"touch /var/lib/apt/periodic/update-success-stamp 2>/dev/null || true";};
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