Sorry if I can't provide an answer on this, but would I be correct in assuming your talking about the idea floated by freshman representative Alexandria Ocasio-Cortez, the policy of having a "70% marginal, pre-deductions/exemptions, tax rate on the portion of a person's incoming exceeding ten million dollars"?

If so, allow me to explain some of the subtleties of what exactly she's asserting was the case, as I think your question is following an, unfortunately common, oversimplified reading of what representative AOC was claiming:

1. The claim is that the uppermost marginal tax bracket was 70% - 90% for every dollar above some large amount. Those who had a large chunk of their income in this bracket would be "The Rich" they're referring to.
2. She was not claiming that the "effective rate" (the rate you pay after deduction and other things are accounted for) was 70% - 90%, only the nominal rate (the pre-deduction/"other tax lower stuff" rate).
3. Expanding on #2, her claim thusly would still hold even if the effective rate was around 40% - 50% (which it probably was), as the point of such high taxation like this is not really to increase government funding (though it does do that), but instead to pressure the rich to do things the government feels would make for a more healthy economy, by tying good behavior to tax incentives, and having a high nominal rate to force business/rich people to take advantage of them.

Mods plz don't delete, btw. I know its not an answer, but I think for his and others' sake, who may not necessarily know the minutia of marginal income taxation, a bit more context would complement nicely an answer.