Geant4 has been removed from the Homebrew package manager due to concerns about the Geant4 license (see https://github.com/Homebrew/homebrew-core/pull/72029, https://wiki.debian.org/DebianScience/Geant4 and https://lists.debian.org/debian-legal/2021/01/msg00014.html). Documentation still suggests Homebrew as a way of installing Geant4 (https://geant4-userdoc.web.cern.ch/UsersGuides/InstallationGuide/html/).
Fixed in the latest documentation for release v11.0