Impacts of Contact Delete on sync between Marketing cloud and Sales/Service

When performing a contact deletion, if the contact is eligible to sync into Marketing Cloud, the system may not recognize those contacts as eligible for sync unless there is an update made to that record or a refresh done to a related data source.

Please also consider the best practice related to contact delete mentioned below:

Understand How Contact Deletion Functions with Synchronized Data Sources

If you delete contacts in Marketing Cloud and a data source synchronizes before the deletion process completes, the process adds a contact record in All Contacts. This record includes an 8-4-4-4-12 character UUID. To prevent this occurrence, delete contact records from Agentforce Sales (formerly Sales Cloud) or Agentforce Service (formerly Service Cloud) before deleting contacts in Marketing Cloud. You can also turn off the data synchronization process for that data source. These new records include the actual Salesforce ID in the ID field of the synchronized data source. The _contactkey field contains the UUID-style contact key used in All Contacts.

If you stop syncing a contact in Marketing Cloud Engagement, the contact is removed from the synchronized data extensions, but not from Contacts. To remove a contact, submit a contact delete request.