This file is indexed.

/etc/update-motd.d/91-release-upgrade is in ubuntu-release-upgrader-core 1:16.04.25.

This file is owned by root:root, with mode 0o755.

The actual contents of the file can be viewed below.

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#!/bin/sh

# if the current release is under development there won't be a new one
if [ "$(lsb_release -sd | cut -d' ' -f4)" = "(development" ]; then
    exit 0
fi
if [ -x /usr/lib/ubuntu-release-upgrader/release-upgrade-motd ]; then
    exec /usr/lib/ubuntu-release-upgrader/release-upgrade-motd
fi