Earlier quoted context omitted.
Slack has a really nice popup that blocks this from happening by mistake, and confirms you really want to do it. (Also, @here is a great alternative because it only notifies people actively on Slack.)
It would be nice if there was something like @eventually for everyone in the channel the next time they get on or similar.
I mean if it's imperative that everyone reads it, just send an e-mail instead or keep a non-interactive #announcements channel.