#1 - May 23, 2018, 2:04 p.m.
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.
As always, thank you for your feedback!