Deflation is not zero interest rates. Redefinition fallacy.
Inflation is not interest rates either.
No one said that deflation was zero interest rates, or that inflation is interest rates.
Normal monetary policy is to increase interest rates to decrease inflation, and to decrease interest rates to increase inflation. That is a bit of an oversimplification, but basically how it works.
The danger is when there is a danger of deflation, but interest rates are near zero. It is difficult to have negative interest rates, so the normal monetary policy cannot be used. One possible solution is quantitative easing. It has rarely been used, but it appears to be the best solution to the problem we found ourselves in during the Great Recession.
So quantitative easing was used, which is where the central bank buys bonds, thereby forcing an increase in the money supply. When quantitative easing has done its job, the central bank stops buying bonds. They can either sell the bonds, or just let the bonds mature. That is called quantitative tightening. We have been doing that for most of the last decade. It has caused some measures of the money supply to go down for the first time since the Great Depression.
Right now, interest rates are well above zero, so conventional monetary policy is possible. On top of that, deflation is no longer a danger.
There is no 'quantitative tightening'. Inflation has been a part of the United States since FDR.
I cannot tell if you mean those two sentences as related thoughts or not. Quantitative tightening is rare, because quantitative easing is rare. It has been happening for most of the last decade.
Modern central bankers feel 2% inflation is the perfect inflation rate. Deflation is the worst, being strongly related to the Great Depression, and even the Dark Ages. The Fed had nearly perfected keeping inflation at 2%, before the Great Recession through everything off.
During the Great Recession(2008-2009) we saw the beginning of deflation, so you are wrong about inflation being continuous since FDR. Even when interest rates went to near zero, they could not break deflation. Quantitative easing barely broke deflation by 2014, but still inflation was below 2%.