During application maintenance or migrations, you can display a custom web page to the user of your application. Heroku serves a static page whenever a user tries to access the application during a maintenance window.
If you need to check the current status of the maintenance window for your application, you can use the heroku maintenance
command. This command will show the current status of the maintenance window, that is, on if maintenance is in progress and off if the application is running in the regular production/development mode:
$ heroku maintenance Off