Inhumans have been removed from the Marvel slate of films for Phase 3. The films isn't officially cancelled so don't worry too much, but it has been put on hold until who know when. 

Kevin Feige gave some clarity to the decision here:

"Since we made our initial phase three announcement, we added Spider-Man, which was a big joyous coup for us. We added Ant-Man and the Wasp, which was a big fun continuation of that story for us. Walt Disney Company has announced an Indiana Jones film for right around that same time. So I think it will shuffle off the current date that it’s on right now. How far down it shuffles, I’m not sure yet."

I think this is just a problem that happens when you announce to many things too soon. Since the first announcement of Marvel Phase 3, there has been the acquisition of Spider Man following shifting dates and now a film removed from the slate entirely. 

I do think the one positive here is that Avengers: Infinity War - Part 2 will now be the closing film for Phase 3, which is a smarter choice in keeping everything fit together. 

Either way Inhumans fans will have to wait a while for this one. But I don think that Phase 4 is already at the planning stages so Inhumans could easily fit into there as Marvel continues to do new films in the future. 

Source: Collider