The --use-pnp flag will break Yarn 2 because PnP has been enabled by default and the flag has been removed. This improves user experience by automatically ignoring the flag for Yarn 2, and explaining that it won't be needed in the future.
I've tested this with Yarn 1 and it works normally, though Yarn 2 has a different installation linking system that I couldn't get to work with create-react-app locally.