There were some areas left on top of Queens / Terminal 2 that you could still access through the 80's and 90's. A purpose built staircase gantry was at one end along with a small canteen/restaurant and there was a small aviation bookshop which I think was the forerunner of the shop that now inhabits what is left of the visitor centre. There was one , very small undercover area with tinted glass there too.
Most of the childrens playground area and the higher of the brick built viewing areas were already shut as being unsafe.
This reduced area through the 80's and 90's was accessed through what I think was supposed to be a fire exit door in the side of Terminal 2 before you went up many flights of stairs to access what was left. There was an entry ticket machine but don't ask me how much it was then as I can't remember.
9/11 IIRC was the final nail in the coffin and the area was gone for good.