After searching for the third time on how to do this, I thought it would be time to write about this here 😬.
If you have an Azure App Service that for some reason should just display a message to the user, indicating that it isn’t available, you can do this.
I have had several reasons to do this:
single app service host, without a deployment slot and a big db update ( > 10 minutes db hitting a spending limit and no wish to update the limit moving dns names and certs between app service plans (were recreated with a better name)
Create or place a new file in your webroot (wwwroot) named “app_offline.htm”. I usually create it using kudu. The appservice will see this and online serve this file, as long as it’s there. Note: the contents of the file can be anything, including css and images from anywhere else in your wwwroot.