I'm not sure that lorewise the death knights and demon hunters have left the Alliance / Horde. All classes had their own issues in the class halls, but no especific class was expelled from or volunteerly left the Alliance or the Horde... at least from what i remember in legion.
Death knights, demon hunters and warlocks are the "bad boys" from the Alliance/Horde classes, because clearly for them the motives justify the means. But still i believe they weren't cast away. I also have a feeling that the DKs aren't following orders directly from the current lich king, they are just connected to the Ebon Blade, just like all characters to their classes / class halls, but i might be wrong though.
Blizzard did put the death knights and demon hunters in the BfA "refrigerator" as we can clearly see in the available classes of the allied races, especially for gameplay reasons (aka having a specific starting zone locked in time with a leader that raised/united them that no longer exists (in Azeroth for the case of Illidan).
My opinion is that these two classes need to evolve, because without a leader and the motive that created their order no longer exists (fighting back the lich king or ending the legion), it feels that there is no reason for the classes to exist anymore. The evolution theory says that the creatures need to evolve to adapt and survive, or else they might die and be extinguished. For me that is clearly the case of the death knights and demon hunters. They need new motivations and leaderships to keep the recruitment opened and get new recruits that should continue filling their ranks, or else those classes will end up dying with no new characters.
I am also very curious in what direction Blizzard will be taking Calia. And what they are planning to do with Uther's Tomb. @
The-Shan wrote after Blizzcon that Uther's Tomb could be just an artistic addition to BfA without a true motive, after talking to a Blizzard developer. I hope it is more than just a new art criation added to the game, and that a cool story might come out of all this.
However seeing how all this awkward war story orbiting around Sylvanas, i feel i might be disappointed again.
But still, i am very interested in seeing how the Calia storyline will be developed and presented to us. For sure she will be a force against Sylvanas (unless she does the same thing as the undead night elf dark rangers are doing
) Hopefully it won't be a disappointment! *crosses fingers*