By default every application on App Platform has starter domain like {app_name}-{some-symbols}.ondigitalocean.app.

Is it possible to disable this domain in case a custom domain is configured?

Having it enabled for websites could potentially affect search engine optimization as content would be mirrored on different domains. For a static website, it’s not possible to have specific robot.txt files for different domains, so there is no way to disallow indexing of the default domain.