#2 - Oct. 10, 2017, 9:36 a.m.
I just posted this in another thread, but I'll mention it again here:
Any artifact appearances you've unlocked (including Druid forms) will be available as transmogrification options after Legion. I'm not privy to the specifics on how it will work regarding Druid forms, but the developers are keen to make sure you can still use those forms after Legion.
You'll also still be able to acquire appearances you haven't earned post-Legion, with the only exceptions being the Mythic 15 Keystone Dungeon appearance, and the Mage Tower appearances. If you don't earn those before the end of Legion, you won't be able to earn them after Legion - think of them as effectively the Challenge Mode sets of Legion.
Hope this clarifies. :)