The United States ISBN Agency is now assigning ISBNs beginning with the prefix 979, which do not have an associated 10-digit ISBN.
Around 1970, 10-digit ISBNs began being assigned to books, up to 2007. In 2007, to better integrate with UPC, EAN, and later the GTIN barcodes, the 13-digit ISBN was launched. All past ISBNs were place within the 978 'Bookland' prefix.
The newer 979 prefix effectively doubles the capacity of the system to be assigned to new books. Now that books are being assigned with 979 prefix, the 13-digit ISBN is the proper way to identify books past and present.