Do they still exist? How does one get them?

I saw this in the fare rules for a TATL Z fare

Z CLASS FARES ARE NOT UPGRADABLE TO FIRST CLASS. NEITHER MILEAGE NOR CERTIFICATES MAY BE USED TO UPGRADE THIS FARE. PASSENGERS WISHING TO HAVE THE OPTION TO UPGRADE SHOULD PURCHASE C OR D CLASS PER THE RULES OF THOSE SPECIFIC FARES. Z CLASS FARES PURCHASED ON/AFTER 15 JUL 03 FOR TRAVEL 15JUL 03 THROUGH 29FEB 04 SWEET SPOT CERTIFICATE MAY BE USED TO UPGRADE THIS FARE.