Starting with the Spring ’19 release’, Salesforce will enforce the 18-month retention policy and begin the process of removing field history data beyond 24 months.
Option 1: Purchase Field Audit Trail (FAT).
The FAT add-on guarantees that you have access to your field history data beyond 18 months. FAT preserves data in Field History Tracking and provides a FieldHistoryArchive standard big object for automated archival. FAT also lets you define a custom policy to retain archived field history data up to 10 years from the time the data was archived.
Option 2: Download the data and add it to a custom big object.
Data older than 18 months is available only via Data Loader or the API. Data older than 24 months is also available, but only until the Spring ’19 release. After Spring ’19, Salesforce begins removing data older than 24 months. Details can be found from following link: https://help.salesforce.com/articleView?id=000273692&language=en_US&type=1
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