Some Drupal sites may no longer have any user interactivity or require any updating of content. They have essentially become static. Because these sites still require security administration, an administrator has to continue to upgrade the site with patches or consider removing the site all together. Alternatively, you may want to produce an offline copy for archiving or convenient reference when you don't have access to the Internet. Before simply removing the site, consider a third alternative: produce a static HTML mirror archive of all public pages on the site and replace the Drupal installation.

Source : drupal.org