We know that developer DICE has called Mirror's Edge: Catalyst "open-world."
But there are varying degrees of open-world; some games qualify as sandbox right out of the gate, while others take a slightly different approach.
The Mirror's Edge reboot (for that is what they're calling it ) falls into the latter category. In a recent Xbox One UK interview , senior producer Sara Jansson confirmed that the game won't be completely open at the start:
"It’s an expanding city that will open up more and more as you move through the story. [A] critique of the first game was that it was a little bit too difficult for a lot of people or a bit too frustrating, and we believe that just throwing players into a huge city to do what they want would have been a little intimidating, especially as our gameplay is pretty different from many other games."
Personally, I don't remember Mirror's Edge as being particularly difficult or frustrating, but the casual explosion has forced developers to make games easier and more accessible for wider audiences. And I've got no problem with an expanding city; I prefer it, in fact. Don't forget that Grand Theft Auto III was this way; you had to reach a certain stage of the story before you could get across that bridge and reach the other half of the city, remember? Even recent games like The Witcher 3: Wild Hunt adopt this style.
Mirror's Edge: Catalyst is currently slated to launch on February 23, 2016 for PlayStation 4, Xbox One and PC.