An LEDs lifespan is measured by lumen depreciation because most LED light sources will decrease in light output over time rather than burning out like other technologies. Some LEDs can burn out but most just grow dim over time. The industry measures the decrease of light output (lumen depreciation) to inform consumers of the amount light to be emitted from an LED fixture or bulb.
The Illuminating Engineering Society's (IES) current standard recommends that lifetime of LED lights be based on the point where the product loses 30 percent of its initial light output. This is called the L70, meaning the estimated point where the LED light produces only 70 percent of its initial light output. There are other industry methods for estimating lifetime that take other things into consideration but this is the most widely accepted.
LED lifespan refers to the point at which it has reached 30 percent lumen depreciation. At 50,000 hours an LED burns on a decreased lumen output.