Sure it happened somewhat. That's how they caught Solzhenitsyn and that's the protagonist's side job in Catch 22.

But we're talking orders of magnitude of difference.

In general "they" don't read your mail because they can't even if they wanted to. Slow mail is extremely expensive to do a through survailence on.

That's kind of the point though. Even if they would read your mail if they could, they cannot because of the technological limitations on the medium.

The intention doesn't matter, the fact is that slow mail is private for all intents and purposes.