I think it would be more beneficial for WordPress to have a method to check the plugin (as a developer) to see if it is using any deprecated calls (like a new Admin Tab in the Repository, "Check My Plugin" or something. If a plugin is found to be using deprecated calls, the developer should be notified as such and given some suggestions on how to fix it.
As a plugin developer, I know that it is sometimes difficult to keep up with WordPress changes - especially when you are offering more than one plugin to the WordPress community.
I agree there should be some check, but I am not sure requiring WP_DEBUG testing is really the answer. It also makes it sound like developers are at fault - but we have no control over what WordPress decides to deprecate or replace, or when they decide to do it. You could have a plugin that is clean and free of deprecated calls, and 3 months later it could have 20 deprecated calls because it uses one recently deprecated function.
I LOVE WordPress, and I like to help out the community, but if it got too difficult to offer my plugins to the world, I would stop supporting or developing them for everyone else, and just do it for myself. It takes a lot of time to keep plugins up to date between WP changes - that's why I think it would be better if they helped us out a bit by notifying us that something might be wrong with our plugin - or it might need to be updated.
Thanks for listening.