Whistleblowers generally do not need anonymous public platforms. The generally can go through an intermediary that is not anonymous and that has a reputation for being trustworthy, that intermediary can look at their evidence and perhaps do some independent verification, and then can publish.

Similar for dissidents. They can generally find a party outside the reach of the regime they are dissidents of that also opposes that regime and agrees with the dissidents. They just need a secure way to communicate with that outside party. The outside party can then handle spreading the message on public platforms.