Chris Clay 9/26/18
Forum
So yes, Amonkhet, Hour of Devastation, Kaladesh and Aether Revolt are now removed from Arena, including packs from the store and the ability to redeem Wildcards for cards from those sets.
For now.
Our goal is to ultimately provide a format beyond Standard for Arena, though what that ultimately looks like will likely develop throughout the next year. Creating this new format isn't something we want to rush into, and this lets us go into Open Beta with a much more reasonable amount of initial content. Beyond that... who knows.
Nick Wolfram 4/30/18
Reddit
We are looking into creating an MTG Arena Extended Constructed format that would use a cardpool of every card in the game, and this format would likely be "always-on" alongside Standard.