Leaded gasoline, phlogiston, cold fusion, phrenology, scientific racism, basically the entire history of the progress of science.
But let's also not forget that you're asking for a list of discredited ideas, so you wouldn't have heard of them, by definition, if things are working as they should, because they're wrong and the former proponents have long since abandoned trying to disseminate them to you.
> Deplatforming these people and, importantly, showing social pressure that these are bad ideas and shouldn't even be entertained as valid debate, has shown to work repeatedly.
If all you're doing is defining "work" as "the censorship target was censored" then you're not claiming anything. When you kick these people off of major platforms, they don't cease to exist, they break off like an extremist splinter group and go to a place with no moderating influences to fester. You don't see them anymore, but they're still there, and then suddenly somehow Donald Trump is the President of the United States and people can't believe it and don't understand how it happened.
> In fact, social pressure is one of the most effective tools against hate and ignorance like this.
Censorship isn't social pressure, it's force. Social pressure is telling wrong people why they're wrong, every time.
People who are wrong need to be made right, not made frustrated and angry and yet not only still wrong but now even more convinced that their wrong ideas are right because they now have real evidence that they're not getting a fair public debate.