Keynesian economics is a poor theory

First, it doesnt work with negative rates. Real rates have been negative for a long time because of subprime (loans they expect to lose).

Second, deflation only occurs when the Money multiplier is high. When velocity increases due to eg rental yields increasing and m2 is obliterated then any fall in output must be inflationary.

Keynesians justified government spending at a particular point in the past but the assumptions can easily be flipped to get the opposite result.