Confluence's plugin system allows you to build apps that customize and extend Confluence. An app (also known as a plugin or add-on) is a bundle of code, resources and a special configuration file that can be installed on a Confluence site to add new functionality, or change the behavior of existing features.

If you use the Cloud version of Confluence, you cannot update plugin versions yourself. Atlassian will release updates to you based on their release schedule. As long as you have an active Gliffy license, users with Confluence admin rights can update the Gliffy plugin any time from the Universal Plugin Manager (UPM).

You can check macros usage on pages. Confluence list all macros from plugins and show where it is used. If no macros are used from specific project (Confluence shows what macros are from what add-on), then is there a big chance that add-on is not used.