Suddenly recessions are defined by jobs reports and unemployment rate instead of GDP. Labor force participation rate, though... that's not a thing.
But they're NOT defined by GDP, except as an unofficial rule-of-thumb. Like all such empirical rules, this is not a hard and fast definition, especially in exceptional situations.
In the U.S., recessions are officially declared by the NBER (National Bureau of Economic Research), which defines a recession as “a significant decline in economic activity that is spread across the economy and lasts more than a few months.” The continued strong growth in jobs and extremely low unemployment rate make it clear that the current U.S. economy does not fit the definition of a recession.