Hi Yuri,
"Give Cloud Build permission to deploy apps in your project. When you deploy your app, App Engine uses Cloud Build to build the app into a container and deploy the container to the runtime. Cloud Build does not have permission to deploy Python 2 apps by default, so you need to give permission before you can deploy apps."
However, it is not mentioned when the change of using Cloud Build is roled out to the deployment process. So, I have created a feature request on the documentation to reflect this change on the documentation. I would like to add that, we welcome your feedbacks on different products and documentation. In the future, you may send your feedback directly to the product/documentation team by using the "Send Feedback" button located on the top-right or at the bottom of any page.