Historically, intelligence services did routinely open and read physical letters sent through the mail, on bulk scales.
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...