> no one writes the Haskell specified in the Haskell report anymore, because all the libraries you might want to use use one or more ghc-specific extensions
This is a bit of a misconception. One can still write Haskell98 whilst using a library that use a GHC-specific extension. The library doesn't (have to) force its consumers to use extensions too!
I phrased that a bit wrong: my experience (from talking to the Haskellers in my company) is that Haskellers generally prefer to turn on a bunch of extensions when they write code. So, while you might be able to write pure Haskell98, it tends not to be idiomatic to do so.
This is a bit of a misconception. One can still write Haskell98 whilst using a library that use a GHC-specific extension. The library doesn't (have to) force its consumers to use extensions too!