Field history tracking - data retention

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.

UPDATE: A few months ago I was told by Salesforce Support that the Field history tracking date would not be deleted after 18 months. When after a few months the documentation had not been updated to include this new information, I opened a new support case. The same support agent looked into this again, contacted the escalation team, which contacted R&D, who came up with a new answer:

Tracking history is guaranteed to be retained for 18 months, and can still be possibly removed in the future, with no estimated time, or any prediction date that it may happen.

I was told to make sure to have my own backups in case it gets deleted. On the positive side, the documentation has now been updated to reflect this information.

PREVIOUS ANSWER: I just heard back from Salesforce support and they informed me that the field history data will not be deleted at all. It is actually the documentation that is wrong and the team responsible for this has been informed. (It's only been a day so the old information is still there. I hope it will be updated soon.)

(This only applies if you don't have field audit trail enabled. If you do have that enabled, you may have to check what happens.)