I wouldn't say the "writers" are endorsing the old website, or not endorsing the new one, but whois is pretty telling:

    $ whois haskell-lang.org | grep ' Name:'
    Domain Name: HASKELL-LANG.ORG
    Registrant Name: FP Complete Corporation
    Admin Name: FP Complete Corporation
    Tech Name: FP Complete Corporation

    $ whois haskell.org | grep ' Name:'
    Domain Name: HASKELL.ORG
    Registrant Name: YaleUniversityComputer Science Department Haskell Group
    Admin Name: Galois Hostmaster
    Tech Name: Galois Hostmaster

I think it's fair to say that haskell.org is the "official" website that has the involvement and implicit backing of the old-timers. Also the GHC compiler is hosted on haskell.org. haskell-lang.org is the upstart. Whether it will last is an open question.

The official website of the compiler itself is here: https://www.haskell.org/ghc/