With the boost in popularity of online games that came with the pandemic and everybody staying home more, queues for some WoW Classic servers have become a reality again.
After riding this out for a bit, Blizzard decided to take some action. Four US servers, Herod, Whitemane, Arugal, and Faerlina, have had layering reactivated in order to allow more players to log in.
Layering was the tech that Blizzard introduced to meet the expected initial surge of players hitting WoW Classic when it launched. (Explained in detail in this post.)
Blizzard viewed layering as a temporary solution and removing it was a gating item for the company to unlock Phase 2 of the WoW Classic plan. But now it is back.
In addition, Blizzard has also disable realm transfers to a list of EU servers in order to keep people from changing to servers where login queues are an issue.