So, engine update has nothing to do with iPhone X, you just need to add LaunchImage 1125 × 2436. Done.
And even, I strongly recommend NOT using storyboards, because you will get full screen of iPad Pro 12.9" - 2048 x 2732px , which is don’t needed actually, because without it adaptation its telln system that its iPad Air and just scale up all using built in iOS system, image looks very good. I bet that Apple do some non linear scaling, so it should better look than getting full canvas and anyway upscaling you game resource. Or you have res for that iPad… I don’t think so.