Pinned topics seem like the feature to handle this; they use it here on meta for things like ‘Hey we’re going to switch to Dark Mode 
for a bit’, or ‘Hey we’re merging in an experimental branch and thing might get weird 
’
Other than that, maybe there’s a space for an Intercom-style announcement thing that pops up, but doesn’t show up in the topic list?
But this seems like more work than it’s worth to me (you could probably also finagle Intercom onto your Discourse instance 
)