Alright, just gonna quote the post that goes over it in more detail, where it lists the actual effects of the 9.1 scaling. (Which isn’t the WoD system. It’s just a bare imitation of it.)
The only overlap in WoD, was PvP gear usable (although not great) for heroic dungeons, and affli locks unique stats weight emphasizing haste and mastery so much that their BiS gear for PvP was mixing in a lot of mythic raid with the gem slots on top of it in WoD s3, since when it got downscaled it still led to a lot more damage than the pure PvP gear.
The rest was properly separated. Which we’ll NOT have in 9.1
In 9.1, you’ll still have overlaps for everything pre-duelist and pre-mythic raid difficulty levels. In other words, it’ll affect MOST of the player base. Which WoD gearing would NOT do.