Variety is reporting that director Michael Bay is circling "13 Hours" as his next film. The film is based on the book "Thirteen Hours: A Firsthand Account of What Really Happened in Benghazi" by Mitchell Zuckoff which Paramount Pictures bought the rights for. The website is stating it as fact that Michael Bay said he was done with Transformers but the closest he has gotten to confirming that was in August when he said "There's kind of a new chapter, a new direction in movies I want to make. I have a lot of stories to tell. And it's about flexing new muscles." In a bunch of different ways all he has said is he is seriously considering leaving the franchise but Bay, Paramount nor Hasbro has officially confirmed that he has moved on. Thinking and doing are two different things (even though people often confuse them as one and the same like say Variety's reporters).
So what does this mean for Transformers 5? It depends. If Bay privately has told Paramount and Hasbro he is done, then its safe to assume the producers are moving ahead with pre-production and actively looking for a replacement director with Bay likely getting an executive producer credit. End result is the Summer 2016 date remains feasible (but that window is rapidly closing). However, it could almost mean that Paramount is eying a repeat of Transformers 4 and Pain & Gain Bayhem combo where they agree to pay for a Michael Bay passion product in return to directing Transformers 5.
In the US, the Bengazi attack on 9/11/2012 became a very useful and highly charged political ammunition with stories of cover-ups, conspiracies and more that have literally reached 9/11 attack itself and the Kennedy assassination in scope of stories weaved around it. As a result this is usually the type of story that studios tend to avoid and get relegated to "independent film" status (with the smaller budget that goes with them) so any potential fallout doesn't damage the mothership corporation while still getting the potential profitable benefit from them. A repeat of the TF4 agreement would be a win-win as Paramount keeps a director they consider integral to the success of the franchise and Bay will get to film something that otherwise would be difficult to made at the budget and marketing support he would prefer. Also Bay would get another Brink's truck full of Transformers money. However it would also mean Transformers 5 would be kicked back to 2017 or even 2018.
So basically, despite this bit of news, from a fan perspective nothing has really changed in regards to Transformers 5 and will not change until a director for Transformers 5 is officially announced.