Lore wise Midtown is still a thing. Gameplay wise it does not exist and likely/hopefully won't. As many don't like to hear, adding midtown will not provide any unique playspaces that are not already facilitated in Financial. This is because Financial was grown out of Midtown. Midtown was nothing but a prototype district and all the design philosophies within it were used to birth financial as the final product instead.
I've said such many times on the forums before and you all always downvote me. So go ahead, downvote me again (though it doesn't do anything) for being just the messenger of reality.