It is possible that auto-deletion was requested to be disabled back in the day. Out-of-the-box, yes, the information you have is correct. There are requests that can be made of Salesforce to change back-end org settings (e.g. number of field tracking fields on an object, number of workflows on an object, etc.), but I don't know if this is still possible with the introduction of Field Audit Trails. I found an Idea citing release notes about what I've mentioned above and Salesforce is pushing the Field Audit Trails add-on.
If data storage isn't an issue for your org and Salesforce can't support you beyond 10 years, you may want to consider a custom solution to create records for the fields you want to track. You could also export the data to a data warehouse and get that information whenever you need via lightning connect or odata connection to show your history data in Salesforce.
I recommend you open a case with Salesforce. Only they can tell you the settings for your org. I hope this helps you some.
You can select certain fields to track and display the field history in the History related list of an object. The field history data is retained for up to 18 months.
You can track the field history of custom objects and the following standard objects.
Accounts
Best Answer
Yes, Salesforce Shield allows you to track field history tracking of detail objects. You can track 60 fields.
For more information, refer my blog. Platform Encryption - Things to know before activating Platform Shield
Also refer Secure Your Apps with Salesforce Shield trail