@Gregor Wolf I apologize for the delay in reaching out to you as there was a discussion taking place internally on the matter. The team verified that the Web App SQL template is being removed and will be replaced with a newer template in the future. In the meantime, they are requesting customers to create a stand alone web app using this template and stand alone Azure Database for PostgreSQL by following this guide.
Once both services are created, you can then go to the 'connection strings' blade of your Postgre service and there should be a web app connection string pre-created. Take this connection string and navigate to your web app's 'Configuration' blade and insert the connection string under the connection strings section, which will securely store the connection string in an encrypted format for you.
We apologize for the extra steps needed to complete this process but want to assure you that a new template is being worked on. At this time, we do not have an ETA of when a new template will be available.
Please let us know if you have any further questions or concerns.