What year was the zip code mandatory for first class mail?

It isn't. USPS Domestic Mail Manual 602 1.4:

ZIP Codes are required on Express Mail, Presorted and automation price First-Class Mail, Periodicals mail, Standard Mail, Package Services mail (except single-piece price Parcel Post), all mail sent to military addresses within the United States and to APO and FPO addresses, all official mail (penalty mail), all business reply mail, and all merchandise return service mail.

nless required above, ZIP Codes may be omitted from single-piece price First-Class Mail (including Priority Mail), single-piece price Parcel Post, and pieces bearing a simplified address.