What problem does this feature solve?

The [Vue warn]: Avoid adding reactive properties to a Vue instance or its root $data at runtime - declare it upfront in the data option. message doesn't provide any insight about the component being affected or the property being set. Having those in place would make the deubbing way more easier.

What does the proposed API look like?

[Vue warn]: Avoid adding reactive properties to a Vue instance or its root $data at runtime - declare it upfront in the data option. Property being added: 'propertyName'. <component trace>