/etc/update-motd.d/90-updates-available is in update-notifier-common 3.168.
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 | #!/bin/sh
stamp="/var/lib/update-notifier/updates-available"
[ ! -r "$stamp" ] || cat "$stamp"
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