The term 'historic' is an official designation used to describe any iris over 30 years old. The idea, mainly led by the American Iris Society, was to try to prevent the loss of important old irises. Of course this has led to some debate as now a 'modern' iris from 1989 could be called Historic !
In my experience iris varieties pre 1960's have a distinctive form, indicative of the earlier times. I have used this as a bench mark on this site, although there are some exceptions due to their rarity.