Arena in Hearthstone will only consist of standard cards
— Hearthstone (@PlayHearthstone) February 23, 2017
Blizzard made a big move last year to Hearthstone, only allowing players playing casual and competitive matches to use the few most recent expansion cards, but now, this move is coming to Arena. This will means the previous expansions such as the Curse of Naxxramas, Goblins vs. Gnomes, Blackrock Mountain, The Grand Tournament and League of Explorers will all be removed from the Arena data base.
This has come to a shock to many, considering the Arena card selections are randomly generated, and there can be no abuse of a set card. However, Blizzard has made this move to make Hearthstone easier for newer players deciding to move into the Arena mode, away from casual and ranked play, where they likely so not even know of previous expansion cards.
Blizzard has also tweaked certain card ratios, where basic cards, common cards and neutral minions will have a reduce chance of being selected. Since mage has been long overpowered with strong board clear like flamestrike, the chance of getting the card will be made more rare.
Game director Ben Brode said this change is experimental and may be reverted in the future.