I know when I went in 1975 it was possible to walk all over it; I also know that on a subsequent visit it was closed to visitors (wardens, with whistles, like a swimming pool 
)
But the only time I went back to mainland Greece was in 1976, with now-DH, & he says we didn't go to the Acropolis 
(We also went to Antiparos via Athens, in 1979, but I'm sure we didn't have time then to visit the Acropolis...)
Anybody know for sure please?