everyone is a meta group that encoumpasses logged out users as well, we should simply suppress it from the admin UI.
@jpalermo allow mods and admins to @mention/message trust_level_0 users and you can blast a message or mention to them.
But … warning, this is highly disruptive, I would probably disable the option after I blasted the “absolutely” absolutely critical message to the entire community.
A scenario where this may be worth doing is “my website was compromised, everyone, please change passwords asap”
Almost all other scenarios would be serviced just fine with banner topics or pinned topics.