Bitbucket has the ability to add alerts in the header bar:
However, this is not public API so is not available to plugin developers. It would be great if plugin developers could utilise this location to add their (actionable) alerts for their plugins.
For the purposes of this issue, providing plugins a way to show an alert for an actionable item (e.g. a misconfiguration or authentication issue) would give plugin authors the means to do so without needing to build their own solution.
Alerts (as they are currently used by Bitbucket Server) are actionable items for a user and/or admin; i.e. the user can do something to make the alert go away by resolving the issue it is telling them about, e.g. setting their timezone, or configuring the instance's base URL.
Alerts are not notifications of events/activity that might be interesting to the user, for instance a notification about pull request activity or changes to a repository. These kinds of passive snippets of information are (usually) non-actionable items and are (usually) customised for a user based on their activity.