beta.reactjs.org currently missing /error-decoder (E.g. https://reactjs.org/docs/error-decoder.html/?invariant=421). https://beta.reactjs.org/docs/error-decoder.html/?invariant=421 will be redirected to https://legacy.reactjs.org/error-decoder.html?invariant=421 which becomes 404. https://legacy.reactjs.org/docs/error-decoder.html?invariant=421