Conformal removability is hard

A planar compact set E is called conformally removable if every homeomorphism of the plane to itself that is conformal off E is conformal everywhere, and hence linear. Characterizing such sets is notoriously difficult, and in this paper, we partially explain this by showing that the collection of conformally removable subsets of S=[0,1]² is not a Borel subset of the space of compact subsets of S with its Hausdorff metric. We give some similar results for other classes of removable sets and pose a number of open problems related to removability and conformal welding, using the language of descriptive set theory.