Yeah, but that happened in places like the USSR, not in the free world.
It's not a 20th-century phenomenon by any means.
They were steaming open, photographing mail, then sending them on, and the photos to MI5. They were also responsible for recording phone calls so had presence in major exchanges.
https://www.lrb.co.uk/v37/n07/frances-stonorsaunders/stuck-o...