Choose the Logout Page for Salesforce Users

Direct users to a specific logout destination that maintains your own branding experience after they log out of Salesforce. Or, send them to a specific authentication provider’s page.
Available in: Group, Professional, Enterprise, Unlimited, Performance, and Developer Editions

From Setup, go to Security Controls > Session Settings. Set the Logout Page Settings to provide the URL of the custom logout page. If none is provided, the default is https://login.salesforce.com unless MyDomain is enabled. If My Domain is enabled, the default is https://customdomain.my.salesforce.com.