A change of date was hinted during Kevin Feige’s talk with Collider a couple of weeks ago, but the hint was for a reshuffling of its release date and not an outright removal from the schedule.
The reasons Feige gave was the addition of Spider-Man: Homecoming and Ant-Man and Wasp to the schedule of Marvel films, and parent company Disney wanting to schedule an Indiana Jones film in the near future. As it stood, Inhumans stood out like a sore thumb, coming after the last Avengers: Infinity War installment, making it the last film of Phase Three. If Phase Three is supposed to be the final chapter in the first Marvel Saga, then ending it with the Inhumans would be a bit anti-climatic.
Feige has stated that the film will arrive, someday, somehow. However, the concept was already introduced in the Agents of S.H.I.E.L.D. TV series, and three years would be enough of a slow burn for a movie tie-in in that series. Perhaps, quietly, the Inhumans will be consigned to the small screen and the big screen version of it will just disappear.