GHC has ignored it since GHC 6.10.1 (November 2008) and Cabal has not supported GHC versions before GHC 6.12 since Cabal-2.0.0.2 (July 2017). Given the passage of time, perhaps the field should be dropped from the Cabal Package Description Format Specification?

Be aware that Win32 still uses it (even though it has no effect) and other packages use it because they have simply copied Win32 (for example, mintty and ansi-terminal).

EDIT1: Elsewhere, @hasufell has identified that the discussion above is GHC-centric. In that regard, it is true that the Cabal User Guide still states:

One of the purposes of Cabal is to make it easier to build packages on different platforms (operating systems and CPU architectures), with different compiler versions and indeed even with different Haskell implementations. (Yes, there are Haskell implementations other than GHC!)

EDIT2: There are other examples where parts of the Cabal Package Description Format Specification have been deprecated and/or removed as things have developed:

• hs-source-dir: deprecated (in favour of hs-source-dirs) 2.0, removed 3.0
• extensions: deprecated (in favour of default-extensions) 1.12, removed 3.0
• build-tools: deprecated (in favour of build-tool-depends) 2.0, removed 3.0

EDIT3: GHC's CApiFFI language extension was introduced in GHC 7.6.1 (that is, after GHC had begun to ignore Cabal's includes: field). I have not identified anything in the documentation to suggest that GHC began to pay attention again to the field on its introduction.