We got an email December 2018 that said they were going to start enforcing the data history retention limit:
We’d like to remind you of some important changes coming to Field History Tracking , as you are an admin of an org that uses this feature. As communicated previously, Salesforce guarantees to retain your field history data for up to 18 months through your org, and up to 24 months via Data Loader or the API. 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. If you have data beyond 24 months that you would like to retain, please take action well before the Spring ‘19 release.
NOTE: Salesforce orgs created before June 1, 2011 are exempt from this change.
*Currently scheduled for February 2019; Date subject to change
I have data beyond the 18-month limit. How can I get access to and retain my data?
Data older than 18 months is only available to you via Data Loader or the API. If you have data beyond the 18-month limit, and would like to access and retain that data, you have 2 options:
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. To purchase FAT, contact your Salesforce Account Executive.
Option 2: Download the data yourself 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. Use Data Loader or the queryAll() API to retrieve old field history. Information and examples about using Data Loader and queryAll() methods are available here. To move this data to a custom big object, see the Big Objects Implementation Guide.
Note: Once Salesforce removes data, you cannot use the getDeleted() API to retrieve it.