> highlight what is politically incorrect and indefensible in society

The big one here of course is communism. Arguably the most censored idea in the history of American politics. The government even held hearings to track down adherents.

In some places it's still difficult to have an uncensored discussion about much tamer topics like labor organizing.

> pedophilia

Lolita was frequently banned

> classified government information

This is generally celebrated rather than challenged by readers. The Pentagon Papers, Snowden leaks, and the Panama papers are famous examples.