There isn’t really a source, since you had to be at the event for the talk session with the president of Shaft, Mitsutoshi Kubota. Basically Aniplex had a Q&A session for him and asked about future Shaft projects. Mr. Kubota replied by saying that they have Nisekoi coming in January and Mekakucity Actors coming in April.
For verification all I can say is that the event did happen and I’d have no reason to really lie about it. It says on the event page for rebellion itself that there is a special talk session with the Shaft president.