When were GADTs introduced in GHC?

Question

When were GADTs introduced in GHC? (version + date)

Also, are they still considered a language extension or are they now part of the Haskell standard proper?

Answer 1

GADTs were first supported in ghc-6.4, which was released in March 2005.

They are still a language extension, the latest standard, Haskell2010, has not adopted them (since it was a GHC-only extension, afaik).