This hero class is currently limited to be available only if a character on the same realm is at level 70 or higher. This character must also currently exist for this unlock to remain in place.
When the Death Knight debuted as a hero class, it was locked until a character reached level 50 on any realm, at which point they could then make Death Knights on any realm.
This has since been removed, and DK’s can be made regardless, with the introduction of the level 90 character boost:
_“With availability of the level 90 character boost, we feel the requirement to have a character level 55 or higher to unlock the Death Knight as a playable class is no longer relevant” - some random blue
Despite the content of this statement, Demon Hunter’s still debuted with a level 70 unlock requirement. While this is fine (TBH I think the DK unlock should still be in place*) in of itself, I do think that limiting the unlock to one realm is a bit much - especially for those who have already maxed the character capacity for their preferred realm. This is why I wish to suggest a cross-realm unlock, at least for the connected realms.
*New players with no game experience having high level characters leads to chaos and stupidity. IMO level boosts are also a bad idea, but as they cost a whack of money, it’s unlikely many kids will purchase them