Really? I think it's not entirely true. I still get souls from spawning dragons.
From spawning dragons yes, just not from RE-spawning dragons that sit on dragon roosts/word walls. Otherwise, you could just fast travel to that local and kill the dragon over and over to end up with 100 souls! LOL
I also don't get souls from dragons I've previously killed. But there are so many dragons in Skyrim, I really don't care.