I have never understood why these things aren't removable by default. Especially when it's something that some users will literally never interact with, not even a single time. Why not make everything optional? I don't get it.
I removed this button with userchrome.css on day 1, but ... I really shouldn't have to.
Blizzard make money extraction software now, not games. The lifecycle of their products starts with a complicated system of overlapping, interrelated components like events and currencies and battlepasses and sales and shops and services and items and subscriptions, and then they dress it up to look like a game.