Given that 70-80% of people in the US will inherit nothing under the current system, any inheritance tax largely only affects those that have generations wealth to begin with. The exceptions might be things like the family farm as small farmers trends to be rather can poor, but with our current healthcare system, I don't think it is likely they could hold onto the farm until death unless they die by suicide or farming accident.

I don't think anyone legitimately calls for a flat 100% inheritance tax across the board. But if that we're the case then it wouldn't be regressive since it's affecting everyone equally regardless of the wealth they stood to inherit. In practice, the wealthy would have a lot of incentive to hide assets through illegitimate means or loopholes which would make it effectively regressive if the less wealthy do not have the means to do the same.

On the other hand, a 100% marginal rate make more sense as an interpretation of what you're saying (I haven't heard it suggested so I'm just guessing). eg. After 20 or 50 million or whatever threshold the rate is 100% to avoid generational "empires" from being passed on. In that case, that's just a very progressive tax.

In both situations, the tax can be used to lessen wealth inequality but the first is a bit more of a social decision instead of just a means of reducing inequality.