This needs to be implemented. Modern is great, but while you're in the process of migrating and upgrading everything, there will be solutions that do not work in Modern/Mobile. For example if you had previously customised a form with JavaScript, when you load the list items in the app, it displays in the Modern view and your custom code is ignored. This is a pretty major risk and there should be a way to block a site from loading in the app completely until it is fully ready for the Modern/Mobile experience.
Peter
This needs to be implemented. Modern is great, but while you're in the process of migrating and upgrading everything, there will be solutions that do not work in Modern/Mobile. For example if you had previously customised a form with JavaScript, when you load the list items in the app, it displays in the Modern view and your custom code is ignored. This is a pretty major risk and there should be a way to block a site from loading in the app completely until it is fully ready for the Modern/Mobile experience.