The 'Marketing Suspend Cause' field is not available as an option in the Change Data Value flow step, or in the Smart List of a Smart Campaign, nor is it possible to manually update the field in the lead record.
Unlike the 'Marketing Suspended' field which is not system managed and can be manipulated by the user, the 'Marketing Suspended Cause' field is a System Managed field and is not able to be updated by the user. The intention of the field is to provide additional details about why a record may have been marked as "marketing suspended" based on a background process.
Note: The "Marketing Suspended Cause" field is no longer used by any background processes. It has been retained as a legacy field to preserve any information that might have been written to the lead record in the past.