I think your comment is purely based on propaganda.
The comment is false on so many levels. Maybe be it describes the 30's and Stalinism but nothing like that was from 50's and onward.
Closed cities were a thing, but they weren't prisons. And "you would end up in the forest with a bullet in your head" certainly wasn't a thing from the 60s on.
Prison for your political view was a common thing until about 1986.
And a lot of people died there.
Forced institutionalization would be far more common than prison for political dissenters in 1970s already.