I cant find that anywhere. Where is the evidence of this?
In GW1 all the classes were pretty much connected to the Gods for the source of their magic and power. This included Warriors. And the Dual Class nature of the first game would imply that magic was never off limits to Warriors. So why is Magic such an offlimit thing to Warriors 200 years later?
Many different cultures had their own Warriors, some of which used different kinds of magic. Dervish for example were stated to also be Warriors.