The Salesforce CLI system returns the Custom Field Metadata XML file without a "trackHistory" entry if it is set to false. This XML entry is optional in the Custom Field metadata type. However, once we deploy such metadata to the org and execute compare between a branch and the org again, it shows differences, saying that "trackHistory == false". But is "false" by default. So I would recommend you ignore this entry if SF Org returns FALSE and the VCS file does not have this item.