/etc/update-motd.d/91-release-upgrade is in ubuntu-release-upgrader-core 1:16.04.12.
This file is owned by root:root, with mode 0o755.
The actual contents of the file can be viewed below.
1 2 3 4 5 6 7 8 9 | #!/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
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