It appears as though Deus Ex: Mankind Divided could see a sequel to Deus Ex: Mankind Divided after all. The sequel was rumored back in September 2016, around the same time Deus Ex: Mankind Divided released.
In the latest episode of Podquisition, host Jim Sterling spoke on the Deus Ex: Mankind Divided sequel that is apparently still in the works. According to Sterling, Deus Ex: Mankind Divided and Deus Ex: Mankind Divided's sequel were in development at the same time. The sequel's development was slowed when some of the developers working on the game were pulled to help with Deus Ex: Mankind Divided.
Sterling noted that the idea of Deus Ex: Mankind Divided's sequel still being in development should be taken with a grain of salt, as there has been no verification from the developers and it's unclear how old the information he has is.
Deus Ex: Mankind Divided received fairly positive reviews which makes the idea of a sequel not too much of a surprise, especially with the game having sold fairly well at launch.