WordPress does not allow you to backup or download the plugins before you jettison them from your site.

All files are removed from the server instantly. After deactivating and deleting them, they simply disappear. Once installed, all plugins list themselves inside Dashboard -> Plugins page. As a site admin you must be aware that you can search and install WordPress plugins directly from the dashboard, or you can upload the plugin as zip files, assuming you received it from another site (for example, a commercial plugin). It may sound simple, but this feature plugs a hole in current set of WordPress plugin functionality. WordPress download plugin is a utility plugin that does one thing, and does it perfectly.

Must-have utility plugin that allows you to download any plugin directly from WordPress Dashboard!