Revision 1 of the x-callback-url draft is now posted. Only a minor change, removing the “version” parameter from the URL path. It was pointed out that having the version in the path served little purpose since a calling app had no way to determine which versions were or were not supported by a target app.
We are now recommending that if your x-callback-url API requires versioning, that it be done by registering different URL schemes with the iOS for each version and handling that internally. The example app on Github has been updated to reflect the change as well.