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I'm fairly sure there's a way to force GHC to compile "Haskell 98" code, which is the first Haskell standard and has stayed the same for 17 years now.

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Nope. There have been many changes since Haskell 98: for example Applicative is now a superclass of Monad. So not only could you not force a current GHC to compile Haskell 98; in addition, you cannot get Haskell 98 to compile on a current GHC at all.

The only way to be sure you can compile old Haskell is to keep an old GHC and old libraries around. Stackage should make this easier going forward.

Applicative didn't exist in 1998, so 1998 code won't have any Applicative code that conflicts with the new language spec.

The problem is that 1998 code will have Monad instances that do not have Applicative as a super class.

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