If you're talking about things like UndecidableInstances or FlexibleContexts, um, they are n't exactly uncommon!

In any case, saying that Haskell 98 is the One True Haskell(TM) isn't accurate. Almost all code (beyond the LYAH level) written today uses a few Haskell 2010 extensions, mostly for ergonomics (things like OverloadedStrings are a good example of this), not to mention all the extensions that make mtl let you write "lift whatever" instead of "lift . lift . lift ..." and infer how "high" you want to go up the monad transformer stack.