Let's not pretend that things are materially worse than they were back then. We certainly aren't blackmailing civil rights leaders into suicide anymore within the FBI.
King letter only came to light because citizens stole it from an FBI office. Maybe the FBI just has tighter security now.
Radical activists today still face state suppression. I know of a communist in Austin who had an FBI file because he led lots of actions, which (I think) he discovered through FOIA requests. In 2018, the Austin PD colluded with two people who claimed this guy assaulted them while he was armed, which was completely false. He's in prison now.