On June 16th, Blizzard community manager Kaivax confirmed that a change to World of Warcraft in how addons interact with Auction Houses went into effect, slowing down how quickly they interact and can run queries.
This change wasn't previously communicated in the patch notes, but in a forum thread users started having their suspicions. Community Manager Kaivax posted a response confirming this suspicion, saying, "we’ve implemented a new system that effectively gives each player a “budget” of AH actions per minute, and only kicks in once that budget has been exceeded."
It should be noted that this won't affect new players, or players who don't utilize any addons. This is purely meant to "throttle players who are using addons to run rapid queries to scan the AH for specific goods, or buy and relist huge quantities of items." Blizzard thinks that by using addons, a small number of players are gaining an unfair competitive economic advantage. This change is intended to reduce the disparity between these players and the rest of the community. Additionally, the constant activity of search queries on the Auction House by these players puts additional strain on the World of Warcraft servers, and by putting this throttle in place it should reduce the strain.
It's definitely strange that this wasn't communicated by Blizzard in the recent patch notes, and that we have to rely on eagle-eyed players in order to get this news. To stay on top of other World of Warcraft news, check out the latest preview on new Shadowlands zones and the recent changes made to Preserved Contaminants confirming a weekly rotation.