At this link https://github.com/tigase/beagle-im , the command to download the Beagle IM is given as:

brew tap tigase/tigase
brew cask install beagleim

This command returns the following error in latest version of Mac OS.

Error: Unknown command: cask

I entered the following command which worked and download Beagle IM in my system:

brew tap tigase/tigase
brew install --cask beagleim

Source: https://github.com/Homebrew/discussions/discussions/902#discussioncomment-398348

So I request you to update the docs.