My newish Discourse installation shows a special message above the forum only visible to staff. It informs that it’s in bootstrap mode for the first 50 users.
How do I use that message dialogue space myself and make it visible to users?
There are certain administrative messages I want everyone to see when they visit the forum. For example when announcing downtime for maintenance.
Press the admin wrench on a topic, choose “Pin” then “Banner”
Also settings > other
option called global notice
Actually, I believe the “bootstrap” style of warning is most easily replicated with the global notice site setting, yes?
The banner message is good if you want links or multi-line kind of content.
The global, non-dismissable notice should be very rarely used, only in world critical urgent cases like
We are shutting this site down permanently
or
We’ve been hacked! Please change all your passwords!
What’s the excuse for the global non-dismissable bootstrap notice then? 
Bootstrap dismisses itself one you’ve met the criteria, e.g. minimum number of posts, users, etc. That particular message is only visible to staff as well, not world-visible.