Homebrew recently deprecated homebrew/versions in favour of making versions available on homebrew/core via the new formula@version format. For example (as per this answer), you can do brew install postgresql@9.5.
Of course, this doesn't work for arbitrary versions. For instance, install cocoapods@1.1.1 turns up "Error: No formulae found in taps".
Under the old method, I could run brew versions <formula> to see available versions. How do I list available versions now?
You can search versions using brew search.
For example:
$ brew search postgresql
postgresql ✔ postgresql@9.4 postgresql@9.5