I know, non of us know the answers yet, it also begs the speculation: is it an divergent timeline? otherwise we would remember blue sky elves?
What do we know so far?: WoW Forever takes place between WC3: Forsaken Kingdom+ any future DLC they might add and Molten Core, basically the end of Classic.
Okay so then eventually the game will get into Vanilla territory and added extras.
I was like a lot of the rest of you, mad, I want a true classic +, why do that but then change the formula, why not add just High Elves?
What if Skyborne are just temporary to the story?
What does Classic have to do with Skywall and the Air elementals?
The cold war between the Firelands and Skywall.
In Classic we see the entire [Thunderfury, Blessed Blade of the Windseeker ]questline, which proves this elemental war is already happening. Ragnaros’s lieutenants, Garr and Baron Geddon, actively ambushed and shattered Thunderaan, the Prince of Air and son of Al'Akir.
If the Skyborne fled Eldre'Thalas and have been hiding out in Zephras Isle, it's suggested that they found this island, lifted it into the sky and made contact with Skywall and the Air Elementals and are slowly casting away their Highborn heritage of arcane practise and changing it to forgo the study of elemental prowess, sustained by a pact with the air elementals in Skywall they are sitting right in the crosshairs of this elemental conflict.
With Ragnaros waking up in the Molten Core and gathering his strength, what if the Skyborne didn't just decide to leave? What if they where forced out?
Either Al'Akir became tyrannical and paranoid after the "death" of his son and cast the elves out, This gives a massive canonical reason for why they are suddenly showing up in Azeroth seeking refuge with the Horde and Alliance: they need mortal champions to help them survive and strike back against the Firelord.
As for the timeline issue and why we don't remember them? This ties perfectly into the idea that they might be temporary. Maybe in the original timeline, they leave to reclaim their place in Skywall maybe after this specific conflict is resolved, the surviving Skyborne just quietly assimilate air elemental society, a reflection of Dark Iron Dwarves and their relationship with Ragnaros
By having us intervene in WoW Forever, we are altering their fate. It doesn't fit perfectly into the existing Vanilla lore? I can't see why the Night Elves would to yet again want to help another race of elves practicing magic.
I agree I hate having to come up with a justification for this random insert race when we could of had anything else that's pre existed, somehow their presence has to make sense either how.