Appeared to have been a bug?
I got the announcement message in my inbox:
Set your rooms’ culture! Owners can now add guidelines to manage expectations for new users. Learn more.
The message is clickable & the "Learn more" leads me to expect that it would link to someplace with info. Previously, it instead linked back to my inbox:
https://stackexchange.com/users/918168/pikalek?tab=inbox
I wondered why it didn't link it to this announcement post instead - it now does so.