Some other time you may want to look at:
Site Settings – Look and Feel – Tree View – Enable Quick Launch.
Source: SharePoint 2013: Quick Launch is missing under Look and Feel in Site Settings
Collaboration, Messaging, Security …
Some other time you may want to look at:
Site Settings – Look and Feel – Tree View – Enable Quick Launch.
Source: SharePoint 2013: Quick Launch is missing under Look and Feel in Site Settings