Eidos Montreal previously developed Deus Ex: Human Revolution (2011) and Deus Ex: Mankind Divided (2016), which were both extremely well-reviewed and Human Revolution sold very well as well. Mankind Divided underperformed according to Square's expectations and the studio moved to developing Shadow of the Tomb Raider (2018), and Marvel's Guardians of the Galaxy (2021). Eidos Montreal is currently providing assistance to Playground Games in their reboot of the Fable franchise for Microsoft.
It is unknown if the new game will continue the prequel story of Adam Jensen that began in Human Revolution and was left unresolved in Mankind Divided, or will be a sequel to the original two games, Deus Ex (2000) and Deus Ex: Invisible War (2003). It might even be a modern remake of the original game, which remains one of the most highly-acclaimed video games of all time but could do with a makeover in terms of graphics and UI (although, strictly, only if the original game's insane freedom and branching story are kept intact).
As the game is in its earliest stages and the last Deus Ex game took five years to make, it might be a fair while before we hear any more about the project. At least it's good to know the franchise will continue, eventually.