Why is so little known about the 1930s coup attempt against FDR?

Because the people who plotted the coup bought ink by the barrel

Perhaps one reason for the heavily-guarded FDR inauguration was that 2 weeks earlier there was an assassination attempt that killed the mayor of Chicago, who was standing next to FDR at the time of the attack.

Is the article saying executing plotters in 1930 would have effects on the type of ppl plotting in 2020?

Imho Trump and Jan 6 would not be possible without the attention capture content amplification machine the tech world built. So all these stories about how things will be much better if grifters are locked away, don't matter since the machine behavior hasn't changed much.