I were doing the lunar festival farming and then remember how I used to get back to this place back in Cata beta. I then decided to retry it and it does still work. This is not very hard to do, but a "small" flying mount is recommended. I didn't find any post about this way of getting in. I don't think this can be used for anything but exploring. It looks like Blizzard decided to simply put a bubble of collision around that spot and leave it unfinished. At some point, you have to land to get passed that "bubble", you can see it in the video. Getting into the uninstanced Stratholme will get you stuck in there.
Video: