https://www.polygon.com/2017/11/3/16604356/world-of-warcraft-level-scaling

Following the announcement of World of Warcraft: Battle for Azeroth, the MMO’s next expansion, at BlizzCon 2017, Blizzard got into more details about what’s coming next for the game. The studio revealed details during a World of Warcraft panel following the convention’s opening ceremony, but not all of them were about the expansion.

At least one major change is coming to World of Warcraft before Battle for Azeroth launches, and it has to do with the recently introduced level scaling system. First revealed in the last expansion, Legion, level scaling allows for all of the zones in that expansion to be relevant no matter what level players enter them at. Blizzard is expanding that system to all of Azeroth and the other areas that are part of World of Warcraft.