SP bought a few hundred 40 foot, 100 ton boxcars for copper service in the 1970s. Almost all were still in service in the 1980s.
Those cars are still in service today I believe, although some of them are marked NOKL now. They're modern design outside post cars with 10' doors and not really the same thing as the classic 40' box that's really the topic of discussion. Also, given their assigned service, the probably wouldn't really be seen outside of one or two routes. They are neat cars though.
I have never heard of a 20 year rule. The AAR Interchange rules limit cars built before July 1, 1974 (if I remember correctly) to 40 years of interchange service. It does allow for programs to extend the interchange life.
When did the 40-year rule come into effect? There were some pretty old cars still in service in the late 1970s at least...