Probably dumb question but, I understand Darnassus is no longer in the game/destroyed. However, I can still go there and it’s fine? I’m not on Classic.
Essentially it’s a time shift. You can visit Darnassus before all the other events that happen (I won’t say anymore as to not spoil it).
The wow timeline jumps around from expansion to expansion. It’s not a linear time line running straight through all expansions. So, a zone may be part of two timelines. Within the zone will be Chromie, a dragon in human form, who can flip the zone from one timeline to the other.
Badlands has a portal to Burning Crusade Daraenor - after it was rent asunder.
After you level some, and also do some WoD starter quests, that portal in Badlands changes to take you to the Warlords of Draenor version of Draenor - when the land was still whole.
At the entrance to Badlands, Chromie sits on a rock and will change Badlands, and it’s portal from one version to the other upon request.
Chromie also supports two versions of Darkshore/Darnassus, Theramore, and Undercity.
We most often use Chromie in SW/Org these days for picking a timeline to level 1-50 in, but her ability to manipulate time has been part of the game for quite a while.
The NPC that switches the individual zones back & forth is Zidormi, not Chromie.
There are several zones where you have an option to choose which timeline is active for it. As Tureja mentioned, the NPC Zidormi can be talked to in the zones (or sometimes just outside them as Nok mentioned for Blasted Lands.) Her location is marked by a speach bubble on the world map. (If you can’t see it in a zone she’s in, it means the character doesn’t qualify for one of the phases, sometimes due to your level and sometimes due to which quests you’ve completed.)
For Darkshore and Teldrassil, she’s a bit south of Lor’Danal in Darkshore.
The actual events of what happened in game were a pre-patch only event for BfA, so they can’t be played through anymore. The story of what happened is available in the form of two Novellas available to read on this site. A Good War tells the Horde side, and Elegy tells the Alliance side.
I will say that the events are pretty important story-wise for Shadowlands (as well as BfA), so if you’re into story, I’d give them a read before starting that content. They won’t really spoil anything before BfA and Shadowlands.
I keep both Darnassus and Undercity in the past. It’s far more convenient that way.
If you chose a Chromie time prior to BFA they are still going to be there when you go past, after BFA you have to talk to Zidormi whom you can spot with a white light surrounding them, and on your map. You can then ask them to show you an earlier time.
You can also choose to start a character in the older zones, which will result in the two cities still being there.