This is just complaining about naming at this point. An "extension" must necessarily be extending the language with something. That something is a feature. When

This is just complaining about naming at this point. An "extension" must necessarily be extending the language with something. That something is a feature. When the GHC devs add a new feature to the language, they put it behind a flag and call it an extension. This is the only way features are added to GHC Haskell.