The OP here is fixed:
https://github.com/discourse/discourse/commit/9a94d1b2126a8722089114ff67b82f2d0564f5ce
everyone is no longer visible anywhere in the UI, it is used internally only.
@trust_level_0 though is a group of all active users on the site, so if you are itching to spam everyone you can aduge the mention settings there.