Fischer Black published his paper in 1995 and mentioned that the most recent time that the USA had experienced the zero lower bound was the Great Depression.
In contemporary macroeconomic terms, the concept, shadow irate, refers to an expansionary monetary environment during a period of zero ZLB.
In such a period, when the central banks implement additional expansionary measures such as quantitative easing (QE), the monetary environment is actually more expansionary than implied by the nominal interest rate alone, and it is possible to calculate what nominal interest rate corresponds to the monetary environment.
Due to the option effect, the shadow short-term rate cannot be observed directly in the market.
Economists use models to infer its value from its effect on longer-term interest rates in the yield curve.