I can confirm this. My pre-snapshot nether fortress is not spawning anything outside of the normal nether spawns - no skeletons of any kind, and no blazes either aside from the blaze spawners. The wither skeletons on their own I might brush off as a rare spawn - but fortresses in my experience are typically lousy with blazes, so it made the change noticeable.
Robert Sayre
I can confirm this. My pre-snapshot nether fortress is not spawning anything outside of the normal nether spawns - no skeletons of any kind, and no blazes either aside from the blaze spawners. The wither skeletons on their own I might brush off as a rare spawn - but fortresses in my experience are typically lousy with blazes, so it made the change noticeable.