ACTION: Remove unused themes and inactive or unused plugins from your WordPress website.
Removing unused themes and plugins reduces potential security vulnerabilities at your WordPress website and is also considered a best practice. It also means there is one less thing to worry about when it comes to properly managing a WordPress website.
Removing unused themes and unused plugins, even if they are not active, means there are fewer targets for hackers trying to exploit weaknesses at a WordPress installation.
Removing unused themes and plugins also frees up file storage / space on your hosting server. Maybe not huge amounts, but who needs clutter, right?
While unused themes and plugins are unlikely to have any impact on your overall hosting performance, it is still considered a best practice to remove them.
Before deleting any unused themes or plugins at your website be sure you have a complete backup. While it is very unlikely that removing these will create any issues, it is still considered a WordPress management best practice.
More Information: https://codex.wordpress.org/WordPress_Housekeeping