Update to Retention Limit for Field History Data
How: When field history data reaches the 18-month limit, Salesforce retains it for 6 more months, but the data is accessible only via Data Loader using Export All or the queryAll() API. After Salesforce deletes data, you can’t use the getDeleted() API to retrieve it.
To guarantee that you can access your field history data, purchase the FAT add-on. FAT lets you define a policy to retain archived field history data up to 10 years from the time the data was archived. To purchase FAT, contact your Salesforce Account Executive. For help retrieving your field history before it’s deleted, contact Salesforce Customer Support.