Dark rangers are Undead elves. First they were only High Elves, but after BfA also Night Elves.
Those Undead elves were most loyal friends of Sylvanas. They did most hardest tasks. Like - they were trying to kill Saurfang e.t.c. Also Delaryn was fighting against her own race, against those who was fighting for their Homeland, for freedom!
In other words - a lot of most trusted war missions!
The war is over. All Sylvanas loyal goblins and forsakens were thrown in to jail. Some of them are walking in chains in Orgrimmar.
But what about Dark Rangers? They are set free! Alliance / Horde does not count them as Sylvanas loyal!
But why? Why most loyal soldiers are not counted as actually loyal?
Good chances they are loyal to Sylvanas and left Orgrimmar at the first chance they got. They have zero reasons to stick to the current Horde as it is, since they all died together side by side with their ranger-general.
Yes, at the beginning there was only Delaryn + 1 Night Elf + 1 High Elf dark rangers +Lilian Voss. All of them were Sylvanas loyal. In fact Lilian Voss was a key character in War missions, finding bodies to rise!
It was little bit updated. Now there is a group on Night Elves together with Blood Elves. About 10 rangers from both races.
There should be 1-2 Night Elves dark ranges in the word, but Blizzard don’t care about numbers. We can probably count that there is a big army of Night Elves dark rangers on the Horde side.