Here, I am explaining how to remove the Notebook menu from Quick Launch menu. Notebook is the default feature of SharePoint online. When you are creating a Site collection in SharePoint online, the Notebook option is automatically visible in the homepage.
Step1
Go to Site Collection -> Site Settings.
Step2
Go to Manage site features under Site Actions section.
Step3
Go to Site Notebook and deactivate it.
Step4
Deactivate this feature.
Result