Blizzard confirmed via the Official Overwatch Twitter account that the dance emotes will be unavailable the Overwatch anniversary event. This also means it will not be purchasable or unlockable after the event is over much like the other seasonal events.
Those who collect any of the 24 dance emotes before the event ends will still be able to keep them.
The dance emotes are Overwatch Anniversary event items.
— Overwatch (@PlayOverwatch) June 6, 2017