In my experience this is a necessary feature, because if there are any issues like this one https://github.com/OctopusDeploy/Issues/issues/5924 that takes longer than a day to fix, an alert message will tell the users the current status.

Examples:
<Yellow alert>The server will be upgraded to 2019.10.5 on 10/31/2019. Please contact nn@nn.us if this disrupts your deployment</Yellow alert>
<Yellow alert>The server has been upgraded to 2019.10.5 on 10/31/2019. Please contact nn@nn.us if there are any bugs</Yellow alert>
<Yellow alert>The server version 2019.10.5 has a bug (see <url link> for more details). We are working with octopus support for a fix.Contact nn@nn.us if you need more information</Yellow alert>
<Red alert>Octopus Deploy cannot deploy projects with referenced variables, we are working on a fix with Octopus support. For more information contact nn@nn.us</Red alert>

Atlassian has this feature available for Confluence and Jira, which has proven helpful in many cases.

Reaching all users through email alone is not enough due to the volume of emails that might received throughout the day.

An example of how the screen can look like: http://help.octopusdeploy.com/discussions/beta-testing-feedback/212

This is sort of what I'm looking for. Would be nice to have certain types of messages such as warning, error or informational which differ in colour?

Maybe even have it team/project specific?

along the line of emailing to teams, for maintenance and upgrades, it's useful to let users (team called 'everyone') know that a maintenance or upgrade event is coming. a custom alert along with an email to a team would be very useful.