Issue is here: https://github.com/yarnpkg/yarn/issues/5885
The problem is that npm just migrated their registry to use CloudFlare. Yarn already used CloudFlare, and had https://registry.yarnpkg.com CNAME'd to https://registry.npmjs.org. CloudFlare has a security restriction that one CloudFlare account cannot have DNS records pointing to another CloudFlare account, so npm moving to CloudFlare broke yarn's existing CNAME to npm's registry (thus breaking yarn).