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.)
What is meta?
- Any Stack Exchange user can ask a question
- The community provides support, votes on ideas, and reports bugs
- Your voice helps shape the way Stack Exchange operates
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.