First up we have patch 7.3.5, which was implemented several months ago as a major shakeup to how leveling was approached. Instead of going from low to high level zones like the way the game had always operated, Blizzard opened up the map with more of a Guild Wars 2 type system -- you now have more of a choice of where to go with a scaling mechanic. But when combined with 8.0's massive stat overhaul, things got murky.
It still will be but it takes up an Azerite armor slot. ST is not as important as AoE when leveling because you want to grab a pack of mobs, burst them down, loot and repeat. For DK specifically, Unholy is better in my opinion for leveling because it has a lot more aoe than Frost with epidemic. Unholy was my second fastest leveling time and I did it on a pre-made character with no leggos or consumables.
The Horde levels 50-60 guide has been rewritten and revamped. My 1-60 Horde leveling guide is now completely updated for the 1.12 Drums of War patch. As you can see it is about 15% larger than the 40-50 guide (single largest guide page yet). There is also a lot of route changes. I moved the entire Silithus section from level 55 to level 58, which allowed me to include a lot more quests for that zone. There is also a lot of other improvement to the route, including adding a lot more quests into the guide, mostly from Moonglade, Silithus and Eastern Plaguelands. This will make the final level 59 grind much shorter.
Something to keep in mind as well is that this will make enchanting materials more valuable, since it is likely most players will now simply scrap their boe greens instead of getting them disenchanted. Also redundant dungeon/raid gear can be scrapped rather than traded to the (guild) enchanter in the run. Less stuff gets disenchanted, meaning there will be less ecnhanting materials, meaning their value will rise. All the more reason to DE rather than scrap as an ecnhanter.