When an administrator deletes a custom field, Salesforce moves it to the deleted fields list for 15 days, during which time it can be undeleted or erased permanently. If the administrator undeletes the field within 15 days, most of its properties and data are restored, except for its field history data, which remains deleted and cannot be recovered. References: https://help.salesforce.com/s/articleView?id=sf.custom_field_delete.htm&type=5