Should you leave inactive plugins on your WordPress website? There is no explicit harm in doing so. You do however want to maintain as clean a house as possible. 

You may want to delete the inactive plugins because, 

  • WordPress will still send you details about the inactive plugins with regards to updates.
  • A plugin is a number of files on your hosting space. Delete it so you can have more space readily available. 
  • As a WordPress security precaution delete any inactive plugins that you do not intend to use.
  • Plugins are still executable files so in case of a hacking attempt these files can get infected or can be used to install malware on your site.

Keep it tidy when it comes to your website. It makes maintenance and troubleshooting so much easier.