The current step was to generalize them so that code that imported Data.List so that they don't conflict with the Prelude versions.
The intent was that once we gained the ability to deprecate re-exports that we'd remove them entirely. We now have a patch that enables us to do such things.
It turns out that if you look at hackage, for the most part folks are actually importing Data.List unqualified to bring into scope foldl' or sort. Almost nobody invokes foldr from it.
The "generalization and subsequent re-export deprecation" path breaks less code than the "leaving them monomorphic and breaking everyone who uses Data.List unqualified" path, even if it leads to an intermediate state that is a bit awkward, and doesn't lead easily to a final state where monomorphic versions of those combinators are available.
-Edward