https://wccftech.com/wow-classic-layering-changes-detailed/

Blizzard has detailed some WoW Classic Layering changes, including a change that has set several servers to permanently operate on one layer.

As covered earlier, the use of layering allows Blizzard to manage launch populations. Classic was released back in August of this year and with players having leveled up and spread more evenly around Azeroth, fewer layers are required per realm.