I have just come back to playing WOW, and am curious with all the changes, is the Demon Hunter quest line still available to the blood and night elf races or is it now part of the character creation stage.
Which Demon Hunter quest line do you mean?
Anyhow, they are both available.
DHs now start at level 8, but they begin in their own starting zone, Mardum, the Shattered Abyss, and they go through all the opening quests as they used to.
The Legion Class Hall and questline is also still available.
I was under the impression that there was a quest that was if your race was an elf you could do the quest (sorry don’t know the name or location) to become a demon hunter
No, there never was such a questline.
It would be neat, and fit nicely with the lore, but no, there never has been any mechanism by which you can change a character’s class to Demon Hunter or anything else.