Black people don't feel pain

makes for a horrifying revelation of the depth of the impunity they feel when they enact violence and indifference on black bodies

That particular falsehood has roots in America's history of slavery. From the same racist doctor who decided that slaves fleeing captivity had "drapetomania":

Dr. Samuel Cartright, for instance, wrote that blacks bore a “[racist slur] disease [making them] insensible to pain when subjected to punishment”

source: https://www.pnas.org/doi/10.1073/pnas.1516047113