Duncan Coutts wrote:

The Cabal package provides the library. The cabal-install package
provides the 'cabal' command line tool.

The deprecated package you're thinking of is cabal-get or cabal-setup.
 

Will Hackage one day provide a way to discover that one package has been superceeded by another?

Currently you can see when a newer version of the exact same package exists, but (for example) take a took at how many gazillion database packages there are up there. Which ones are active? Which ones are obsolete? How can I tell??