Google is to shut down App Maker next year, but is asking users to review it in their domains and to take action accordingly.

Google App Maker will be turned down gradually over the course of 2020 and officially shut down on January 19, 2021.

Today, existing apps continue to work. Though App Maker is no longer under active development, the service will continue to be maintained.
Starting April 15, 2020, you will no longer be able to create new App Maker apps. You will still be able to edit and deploy existing apps.
Starting January 19, 2021, existing App Maker apps will stop working and you will no longer have access to them. App maker data stored in Cloud SQL will remain unchanged and continue to follow the policies established by your Google Cloud Platform (GCP) account.

[gsuiteupdates.googleblog.com...]