While in earlier Summer '15 release, we can use Salesforce Sync desktop agent, create folders and subfolders created in the desktop, and it will sync to Salesforce as folders too, this works for both Classic and Lightning. But same as the folder in Library, you need to click each folder to see subfolders underneath.
This screenshot was taken from Classic.
Folderize provides the ability to create folder hierarchies of unlimited levels, drag files or add library documents to the folders. It provides a view of all folders and sub folders in a glance, and work in both Classic and Lightning. You can add documents to Folderize from both library and files.
Type: Managed Package; AppExchange
Blog last updated: 27 August 2017