Well, void is too strong a word. Nothing can completely void your warranty, but lots of things can be the basis for warranty service being denied on certain parts. If you run 15-50 and you lose a bearing in the motor it is conceivable that SUbaru might decline to service it due to the thicker oil being used.
The problem w/ 15-50 is w/ flow - especially cold flow. Cars designed to run on thinner oils often have smaller passages for the thinner oil to flow through. Trying to put a thicker oil through those smaller passages could result in oil starvation of certain parts of the engine resulting in failure.