As of the release of MacOS 11.0 earlier this week, Apple and Homebrew no longer support MacOS 10.13. This means homebrew no longer provides binaries for MacOS 10.13, and on those systems, everything has to be built from source.
osx_image: xcode9.4 is still the default
os: osx image. As a result, many jobs fail because they have to start building all the brew packages from source.