What were the long-term effects on the participants of the Stanford Prison Experiment?
I found this snippet from Quora: Whatever happened to the participants in the Stanford Prison Experiment? 

"They performed extensive follow-ups by bringing all the subjects back individually or in groups over a period of the first year and then for the next ten years they have surveyed them by mail and phone and the consensus is that they did suffer during that week of the experiment, but they learned a great deal about themselves and about human nature that most of them say was quite valuable."

Perhaps within the first decade after the experiment they found constancy in how the participants were affected and felt there was no longer a need for further analysis / recording.