The new way to list all installable cask packages is:
brew search --casks
The man page for brew has the following information:
search –casks
Display all locally available casks (including tapped ones). No online search is performed.
Update:
While the documentation didn’t change, seems like search-text is now mandatory. We can specify * by adding a \* as parameter (to avoid it thinking we are referring to the content of the current directory). So basically it’s possible to do:
brew search --casks \*