There was a 1998 "Haskell Report" which formally defined Haskell-the-language. This version of Haskell is called Haskell 98 (sort of like C++98). There is also an updated version released in 2010, which defines an expanded language called Haskell 2010.

https://www.haskell.org/onlinereport/

https://www.haskell.org/onlinereport/haskell2010/

The purpose of these documents is to specify what an implementation of Haskell 98 or 2010, respectively, should support. Just like the analogous standards published for the C and C++ languages, the Haskell reports make no reference to any specific implementation.

The most popular implementation of Haskell is the GHC compiler. (In fact, in reality it's the only compiler anyone uses, with very few exceptions. For better or worse, Haskell is a one-implementation language at present.) GHC accepts a greatly expanded language compared to what the reports specify, including a large number of language extensions that can be turned on with flags. (sort of like how Clang or modern GCC accept C++11/14). There are minor changes from one version to another, so the language accepted by GHC today is often called GHC Haskell for disambiguation.

What GP is pointing out is that almost everyone using Haskell today writes in the language that GHC accepts today instead of sticking to some official standard.

If you want to learn Haskell, you don't need to worry about this at all. All reference materials and books teach modern GHC Haskell, which is also what is used by practically all Haskell users. As noted before, very few people use non-GHC compilers (usually it's just the people implementing or testing them) and you're very unlikely to encounter these implementations.