TL;DR
brew install carlocab/personal/unrar
brew update したら、こんなメッセージが……
==> Searching for a previously deleted formula (in the last month)...
unrar was deleted from homebrew/core in commit 3f8c7632a6:
unrar: remove (incompatible license)
See https://fedoraproject.org/wiki/Licensing:Unrar
To show the formula before removal run:
git -C"$(brew --repo homebrew/core)" show 3f8c7632a6^:Formula/unrar.rb
If you still use this formula consider creating your own tap:
https://docs.brew.sh/How-to-Create-and-Maintain-a-Tap
直訳すると……
==>以前に削除された式を検索中（先月中）...
コミット 3f8c7632a6 で homebrew/core から unrar が削除されました。
unrar: remove (互換性のないライセンス)
https://fedoraproject.org/wiki/Licensing:Unrar を参照してください。
除去実行前の数式を表示するには、以下のようにします。
git -C"$(brew --repo homebrew/core)" show 3f8c7632a6^:Formula/unrar.rb
もしまだこの公式を使っているのなら、独自のタップを作ることを検討してみてください。
https://docs.brew.sh/How-to-Create-and-Maintain-a-Tap
困ったのでググると、こちらに辿り着き、すでに独自のタップが作られていて……
$ brew install carlocab/personal/unrar
==> Tapping carlocab/personal
Cloning into '/usr/local/Homebrew/Library/Taps/carlocab/homebrew-personal'...
remote: Enumerating objects: 12, done.
remote: Counting objects: 100% (12/12), done.
remote: Compressing objects: 100% (9/9), done.
remote: Total 346 (delta 6), reused 9 (delta 3), pack-reused 334
Receiving objects: 100% (346/346), 105.69 KiB | 301.00 KiB/s, done.
Resolving deltas: 100% (204/204), done.
Tapped 9 formulae (40 files, 178KB).==> Installing unrar from carlocab/personal
==> Downloading https://github.com/carlocab/homebrew-personal/releases/download/unrar-6.0.3/unrar-6.
==> Downloading from https://github-production-release-asset-2e65be.s3.amazonaws.com/310470753/154d6
######################################################################## 100.0%==> Pouring unrar-6.0.3.catalina.bottle.tar.gz
==> Caveats
We agreed to the UnRAR license for you:
/usr/local/opt/unrar/license.txt
If this is unacceptable you should uninstall the formula.
==> Summary
🍺 /usr/local/Cellar/unrar/6.0.3: 6 files, 564.5KB