Square Enix has put all future Deus Ex projects on indefinite hold, following dissapointing sales for Mankind Divided
Deus Ex: Mankind Divided toped sales charts during its release week, despite selling less copies than Human Revolution. Now it seems like the game's overall performance wasn't enough to keep Square Enix interested in continuing the franchise.
At this time most of Square Enix's development efforts seem to be focused around Tomb Raider and an upcoming Marvel game. Eurogame r speculates that there simply won't be enough development resources left over from those two projects to get a big budget Deus Ex game off the ground.
Another thing to consider is the ever more frequent publisher mantra of if it can't start or be a franchise, it's not worth the investment and Deus Ex seems temporarily placed in a victim position to such an approach to business. In any case, fans of the title are likely to be in for a long wait before another instalment to the series arrives.
While the publisher has denied abandoning the series a definite statement regarding a sequel and when it might arrive has yet to appear.