Fact
Skyborne and the Zephras Isle starting experience require a paid upgrade: Skyborne Heroic Pack ($29.99 per Icy Veins/wiki), Skyborne Epic Pack (adds beta access from Sept 17 and 30 days game time), or Warcraft Forever Collection. No in-game unlock path announced.
Confidence note
Official (Blizzard) (pack contents) + Confirmed (multiple credible press, BlizzCon panel/demo) (price)