On June 22, 2017, default inclusion lists were set for all previously existing Salesforce-integrated customers who did not have an inclusion list. The list includes all contacts, except those with a lifecycle stage of Subscriber and will appear in your account as Salesforce inclusion list (default).
Please note: there are no underlying functionality changes with this update. Contacts with the lifecycle stage of Subscriber do not meet the criteria for the default inclusion list and are, therefore, ineligible to sync.
Accounts created after June 22, 2017 will not have an inclusion list by default, and all contacts will sync unless an inclusion list is created for your account. You can learn more about creating inclusion lists here.