Why? The homebrew-core has removed plan9port (see Homebrew/brew#6478 and Homebrew/homebrew-core#38394 ). So we cannot install with brew install plan9port anymore.
Use Github workflow to track lastest commit on master branch of plan9port every day.
If there is a new commit, it would push to update version of this formula.
brew tap tizee/plan9port-rb
brew install plan9port