I sat with event trace on for like 20 minutes because this annoyed the bejesus out of me. I intended to make a simple little addon to block it but…
It shows up as a Battlegrounds Neutral notification. So if you right click chat tab, go to filter settings, Other, and uncheck Battlegrounds Neutral it SHOULD stop.
Of course, Blizzard could just grow brains and realize literally everyone doesn’t need to see that message, but…Good luck.