Well not entirely dead, but according to this article , it's changing quite drastically. What set Windows Phone Design apart is basically being stripped away and it will follow similar patterns as on Android / iOS - as it will be easier for users to migrate to WP.
I'm a Windows Phone user (also use the others) and I can't say I'm agreeing with everything. I love pivots and panoramas and the emphasis on typography. But I'm just one data point...
But I think the main reasons why WP didn't grow as much as it could was because it was lacking the apps, not because of its distinct design language.
I'm now considering to make my iPhone my main phone.
If you're a WP user, what do you think about the coming changes?