I think this kind of access pattern is typically a code smell. Instead of carrying around arbitrarily nested values you should try to normalize it into a known and consistent shape when crossing serialization boundaries.

Proxy pays a big performance penalty, and I doubt it'll be improving any time soon. In addition, I believe it's not possible to polyfill Proxy. An alternative could be to just wrap the deep property access in a try/catch:

    function getQux (input) {
      try {
        return input.foo.bar.baz.qux()
      } catch (e) {
        return null
      }
    }