I completed the BFA Campaign on my main. I expected this zone to be available on my alts.
On my main, I can just fly into the Arathi Highlands and its the BFA version. When I take my alts they are in the cata version. My alts can see Zidormi in Tirisfal and she has the option to go BACK in time to before the Battle for Lordaeron.
Sorry if this is a repost, I tried googling for an answer, but I haven’t been able to find anything definitive yet.
Zidormi for Arathi should be standing by the bridge from Wetlands I think? Should be. Might not be if there’s a bug I guess.
I didn’t see any text bubble on the mini map when I flew my alt out there.
maybe check the area really carefully sorry maybe the text bubble doesn’t show for some reason
I found her. She’s just north of the bridge from the Wetlands, standing next to the sign post.
But she doesn’t have any option in her dialogue.
Well that sucks. I guess that’s a bug. Are you in Cata timewalking? Maybe that would prevent it but I don’t really know. Maybe she’s just bugged I don’t know. Sorry.
The reason why she doesn’t have the bubble over her head is the fact you need to unlock and open up the BFA intro quests (do the quests into boralus or Zandalar until you get the achievement to pop up) and then you can go back to Zidormi and she’ll give you that option. It sucks you may need to do that on all your alts but that is what you need to do. 
Hope that helps!
Welcome to Zandalar?
When my alts arrive on the docks from the portal, Nathanos is standing there with a quest to start the bfa campaign back with sylvanas in orgrimmar.
Are your alts at least level 50?
Good quetsion! im only attempting this on my lvl70 alts.
The answer is you need to unlock the warfront content by whatever questlines. I’m not going to pretend to remember which exactly those are, but I imagine Boralus/Zuldazar is where you need to be (specifically near the boats where the portals and NPCs are).
Have to use the portal when the Warfronts are active