maybe you can try to add and customize an offline.html page
the doc is here :
While you deploy, the app is still usable. There is no direct need to show a message. After the deployment you need to do a restart. Unfortunately, it is not possible to add a custom message or something. Maybe that would be something for the idea forum.
If you want to do maintenance in a running app, you could implement your own maintenance. It could for example show a message on the login screen and deactivate the login for users while you are working on it.
Thank you Andreas!
Do you know if there's an Appstore Module that provides such a functionality?