Hey everyone, the changes made to Arena are not a spiteful attempt to take something away after giving you a nice thing! I’m sorry if the wording around this change has been vague, and I am sorry that this change has had such a negative impact for some of you. 
Unfortunately, the team won’t putting back Class XP into Arena. Prior to 4.2.5, Arena was never intended to award XP/Class Wins. When 3.5 (Classes) released Arena was included in the patch notes to give a general overview of where Classes applied. At this time, Classes applied globally to your Hero, so via a little loophole they applied to Arena, too. To enable the Gem removal cost the Class system had to be redesigned. This means that Classes now apply per team where the Class is in use. As Arena doesn’t have Classes, the ability to attach a Class to your team was inherently removed. As the Arena has a special type of team, technically the new system can’t add a Class. This isn’t something the team is planning on adding again, especially considering this update was a small one.
There’s no statistical benefit for having your class in Arena, so based on the new system and the previous design choice the ability of Class XP to be earned in Arena was removed.