Sadly, Deus Ex: Mankind Divided, in spite of being great, was ultimately a commercial failure. Sales fell far short of sales for 2011’s Deus Ex: Human Revolution, and definitely fell far short of expectations, given the five year long development cycle that Makind Divided went through. And unfortunately, it looks like the Deus Ex series will be paying the price for this failure.
According to report on Eurogamer, it looks like there is no new Deus Ex game in development for now- with Square Enix’s partnership with Marvel has come a dramatic reorientation of projects that their western studios are assigned on. Crystal Dynamics are working on the Avengers game, which means that Eidos Montreal now have the task of developing new Tomb Raider game- in addition to a new Guardians of the Galaxy game, possibly. All of which means Eidos no longer has any manpower to work on Deus Ex for now.
Of course, this doesn’t mean that Deus Ex is done for forever- Square Enix may choose to revive it some day in the future, much like they did with Human Revolution in the first place. But at the same time, given the limbo that franchises like Thief and Legacy of Kain find themselves in, that may not be too likely.