The name is the whole story
The Skyborne call themselves shen'dorei - the hidden people. Two zones away, in Kalimdor, sits the ruin of the people who called themselves Shen'dralar: those who remain hidden.
That is the same word, and it is not a coincidence. Both are Highborne, both come out of the same city, and both answered the same catastrophe by disappearing. One hid underground behind a warded gate. The other hid in the sky.
Forever gives you a starting zone on one and a levelling zone built around the other. They are the two halves of one story, and this is the part that no patch note will spell out for you.
Where they come from
The Skyborne are high elves descended from the people of Eldre'Thalas, the Highborne city in southern Kalimdor that survived the Sundering behind a shield its own arcanists wove.
Surviving is not the same as living well. Eldre'Thalas turned inward under Prince Tortheldrin, and what happened there is its own page - it ends with the prince killing his own people to make a stolen power source last longer. Some did not wait to find out how it ended. They fled.
Blizzard's own panel wording puts them as "rebels who fled Eldre'Thalas after the War of the Ancients and eventually found refuge in Skywall." The fuller account names what they fled from: Tortheldrin's persecution. Both fit - the purge came after the Sundering, which is what the War of the Ancients ended in - but only one of them says why anybody ran.
The pact
What the exiles found was not a country. It was an offer.
The wind spirits of Skywall raised their island home out of Azeroth entirely and anchored it in the Elemental Plane of Air. Not a flight, not a migration - a removal. Zephras Isle stopped being a place on the map.
Skywall is not a friendly address. The titan-forged built it, and they built it for a reason: to imprison most of Azeroth's air elementals. It is a realm of lightning and wind, and its lord for most of recorded history was Al'Akir the Windlord. Whatever the wind spirits got out of sheltering a few thousand refugees, they were not doing it out of the goodness of a plane that exists as a prison.
What the pact cost
Pylons held the island together. For millennia that was enough, which is the kind of sentence that always has a second half.
The wind spirits vanished - centuries ago, with no explanation on record. The pylons began to fail. Parts of the island started falling into ruin and darkness. A people who had removed themselves from the world for safety found that the safety had a maintenance schedule and nobody left to keep it.
Everything that happens on Zephras Isle at character creation happens because of this. The Windshapers answer it by holding to the elements and the pact. The High Order answers it by going back to the arcane their ancestors abandoned. A third group answers it by deciding the Windlord should simply be let in.
That the Windshapers reached the Horde through the Earthen Ring and the High Order reached the Alliance through the Kirin Tor is not a menu decision dressed up as lore. It follows: shamans find shamans, and arcanists find the tower full of arcanists.
What is still missing
Nobody has said where the wind spirits went, and it is the load-bearing question of the whole zone. Everything on Zephras Isle is downstream of their absence.
Also unstated: whether the island can be repaired, what happens to it if it cannot, and whether the Skyborne ever learn what the wind spirits got out of the bargain in the first place.
The practical details - which classes each side unlocks, the level range, the beta findings - are on the Zephras Isle page. This one is about why any of it is happening.