When the Objective-C code above is imported into Swift, the methods are mapped to a true subscript declaration. However, any availability attributes on the original methods are lost…or rather, the getter's availability is copied onto the subscript, but not the setter's.
The relevant functions here are buildSubscriptGetterDecl and buildSubscriptSetterDecl in ClangImporter.cpp; these functions should call Impl.importAttributes to get the correct attributes for the getter and setter. We also need to figure out what's pulling the getter's availability onto the subscript; that's not really correct if the getter is deprecated but the setter is not.
Tests for this would look something like https://github.com/apple/swift/commit/b0e12f4c80790364b7427af981e652f1c1c0da42#diff-348ce3800f609e6140007f774b227469R874.