If there are security issues with themes and plugins that have been deactivated hackers may still get access to them and gain access to your website
Deactivated themes and plugins installed on your server increase the backup file
You don’t have to worry about keeping themes and plugins up-to-date if you delete themes and plugins that you are no longer using.
Themes and plugins can always be downloaded again should you need them.
After deleting a plugin not all the data is removed. Developers want to keep user settings should you re-install the plugin. Bits of data can eventually clutter your server and have an effect on your website’s performance.
How to fix this? Go to your cPanel and ensure that the plugin folder is no longer in the plugin directory. If it is still there, go ahead and delete it.
BE SURE TO BACK UP YOUR SITE FIRST IN CASE SOMETHING GOES WRONG!