Unless I'm greatly misremembering, it's been like that since before MathOverflow moved to SE 2.0.
(I suspect that the site shouldn't be used anymore.)
On that URL, it will never go out of private beta.
That is indeed on the SE 2.0 platform.
I have made a change, so dev sites will actually have a more meaningful and accurate message instead of the beta one. This is in the next build.