In 2006 both the International Monetary Fund (IMF) and the Federal Reserve started to highlight a decade-long decline in the slope of the Phillips curve, i.e. the coefficient of economic slack (IMF (2006), Iakova (2007) and Roberts (2006)).
This decline has been challenged by Gordon (2007 and 2013) as being too dependent on the Phillips curve specification, being particularly associated with variants of the New Keynesian Phillips curve (NKPC), but not verified by Gordon’s own “triangle model”. Coibion and Gorodnichenko (2015) show that evidence of slope decline is  2  BIS central bankers’ speeches  mixed for the United States.