Note that there will be level 85 heirlooms that will not stop working at 80. However, you’ll have to get at least one character to 85 to purchase them.  (Source)

What incentive, aside from a short leveling burst, will there be to get 1-85 heirlooms?
There may not be a ton of incentive if you have a full wardrobe of heirlooms already. We will be making new kinds of heirlooms though, including some new slots. Many players also reach a point where they can’t find anything else to spend badges on, so heirlooms look more attractive then. (Source)

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