This file is indexed.

/etc/apt/apt.conf.d/15update-stamp is in apt-config-auto-update 2.0.0-2.

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 {"touch /var/lib/apt/periodic/update-success-stamp 2>/dev/null || true";};