In case if you wanted to hide "NewsFeed" app from the list of O365 Apps (App launcher), there is no direct way to disable it like the below apps.
We can use the below options to hide or disable the NewsFeed app.
Option 1: To hide it from the Home Tab (Note: This setting is specific to each user)
Option 2 : To Disable Newsfeed for all the users.
• Sign in to Office 365 with your work or school account.
• Go to the SharePoint admin center.
• Select Settings - Enterprise Social Collaboration.
• Check "Use Yammer.com service"
• Click Ok.
• Go to the SharePoint admin center again
• Select user profiles.
• Under My Site Settings, click Setup My Sites.
• In the Newsfeed section, clear the Enable activities in My Site newsfeeds check box.
• Click OK.
• The above changes would take 30 mins to reflect the changes. Logoff and Login back after 30 mins to see the changes.