Eidos Montreal head says Thief is Dead
During an interview with PCGamesN, Eidos Montreal head David Anfossi explained that the Dues EX franchise is on the backburner whilst they are busy completing other projects but they have no intention at all to return to the Thief franchise, he corroborated that statement to solidify a 2017 tweet he made.
Eidos indicated back in 2014 when it released, that future Thief sequels would entirely depend on how Thief was received.
Upon release the critical reception for the game was mediocre and they even backtracked on their initial next gen only status and also released it on previous gen (360/Ps3) consoles but the sales just wasnt there thus a Thief 5 was unlikely from the beginning.
A motion picture adaptation of the Thief Franchise is in the works but there's no director or any primary cast members attached to the adaptation yet so it's eventual theatrical debut is still in limbo.