With a customer I have today seen the first time that there is a feature of custom message banners in Azure DevOps. This can not be set via the UI but is a command line only option. Documentation can be found here
https://docs.microsoft.com/en-us/azure/devops/organizations/settings/manage-banners
# List existing banners
az devops admin banner list --org https://dev.azure.com/myorganization/
# Set a banner
az devops admin banner add --org https://dev.azure.com/myorganization/ --message "INFORMATION: Network domain updates will occur on September 3" --expiration 2019-09-04 --type info