You can write that line better than that, but you cannot get rid of the String?, because AnyKeyPath and PartialKeyPath don't provide their Value to the type system. nil is strongly-typed, and so I'm having trouble imagining where @bdkjones thinks it could be acquiring that.
foo[keyPath: kps.first as! ReferenceWritableKeyPath] = nil as String?
foo[keyPath: kps.first as! ReferenceWritableKeyPath] = String?.none
typedNil(of: type(of: foo.name))
could be rewritten as
type(of: \Foo.name).Value.none
extension KeyPath { typealias Value = Value }
But that's the same as
foo.name = nil