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Artifact Appearance Availability Change in Battle for Azeroth
Blizzard recently made an adjustment to what artifact appearances are going away in Battle for Azeroth. The appearance awarded for completing a Mythic +15 will now still be obtainable after Legion ends. This means that only the Mage Tower base artifact appearances will no longer be available to unlock in Battle for Azeroth.

Originally Posted by Blizzard (Blue Tracker / Official Forums)
As testing continues in the Battle for Azeroth Beta, and we specifically consider the transition from Legion to BFA, we’ve decided to make the existing Keystone Master achievement permanent. There will not be a new achievement for completing a Mythic 15 or higher dungeon in the new expansion.

The artifact appearance associated with Keystone Master will still be available in Battle For Azeroth. Players can earn the achievement in Legion through August 13, or players can complete a Mythic 15 BFA dungeon at level 120, once Mythic Keystone difficulty opens a few weeks after the launch of Battle For Azeroth.

This means that the only artifact appearance that will become unobtainable with Battle For Azeroth is the Mage Tower base appearance.

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