Summary
Sequel Proをアップデートしようとしたら invalid 'depends_on macos' value ...Homebrew2.0.0からは macOS10.8以前がサポート外になり、
macOSのバージョンを表す定数から10.8以前のバージョンが削除されたことが原因。
トラブル例
% brew cask reinstall appcleaner
==> Downloading https://www.freemacsoft.net/downloads/AppCleaner_3.5.zip
==> Downloading from https://freemacsoft.net/downloads/AppCleaner_3.5.zip
######################################################################## 100.0%
==> Verifying SHA-256 checksum for Cask 'appcleaner'.
Error: Cask 'appcleaner' definition is invalid: invalid 'depends_on macos' value: ">= :tiger"
対処法
depends_on macos と書かれている Cask Formula を確認。サポート外のバージョンであれば、当該行を削除する。
% cd /usr/local/Caskroom
% grep depends_on **/.metadata/**/*.rb
adobe-reader/.metadata/15.023.20053/20170212164009.034/Casks/adobe-reader.rb: depends_on macos: '>= :mavericks'
appcleaner/.metadata/3.4/20171014100051.121/Casks/appcleaner.rb: depends_on macos: '>= :tiger'
atom/.metadata/1.27.1/20180528113851.853/Casks/atom.rb: depends_on macos: '>= :mavericks'
epic-games/.metadata/10.5.4/20191004125924.495/Casks/epic-games.rb: depends_on macos: '>= :sierra'
google-backup-and-sync/.metadata/latest/20180528062642.748/Casks/google-backup-and-sync.rb: depends_on macos: '>= :mavericks'
iterm2-beta/.metadata/3.1.6beta4/20180319062833.560/Casks/iterm2-beta.rb: depends_on macos: '>= :yosemite'
mactex/.metadata/2019.0508/20190519140126.061/Casks/mactex.rb: depends_on formula: 'ghostscript'
mactex/.metadata/2019.0508/20190519140126.061/Casks/mactex.rb: depends_on macos: '>= :sierra'
macvim/.metadata/8.1.151/20180815183144.570/Casks/macvim.rb: depends_on macos: '>= :mountain_lion'
mysqlworkbench/.metadata/8.0.18/20191016183046.847/Casks/mysqlworkbench.rb: depends_on macos: '>= :mojave'
virtualbox-extension-pack/.metadata/6.0.12/20190904205437.663/Casks/virtualbox-extension-pack.rb: depends_on cask: 'virtualbox'
Reference
Homebrew 2.0.0 - 2019-02-02https://brew.sh/2019/02/02/homebrew-2.0.0/
GitHub - Homebrew / brew - brew/Library/Homebrew/os/mac/version.rb
https://github.com/Homebrew/brew/blob/372a8d45325f53355c36c0f1b264b72cc186bcb0/Library/Homebrew/os/mac/version.rb#L6
GitHub - Homebrew/homebrew-cask - Issues #58046 Fix casks with `depends_on` that reference pre-Mavericks
https://github.com/Homebrew/homebrew-cask/issues/58046