I stopped watching the Tonight show with any regularity before Johnny Carson left, but I am pretty sure that they used the peacock up front until the very last day he hosted. I think that would place it somewhere in the early 90's that it stopped being used on a daily basis prior to the Tonight show. I agree that it was dropped many years before that for all other shows so I'm thinking Johnny may have had something in writing saying they still needed to use the peacock with his show until he retired.