Historically, Tenable provided offline plugin bundles for customers who needed to update Nessus on machines without an internet connection. This file is essentially a snapshot of the entire plugin database at a specific point in time.

: This URL will trigger the download of the latest plugin set, often named all-2.0.tar.gz or nessus-updates-x.x.x.tar.gz .

You obtain plugins without a paid subscription, but with limitations. Here are the legitimate methods related to the "free" aspect of your search.