As explained above. It will help when a custom field is deprecated, but new users don't realize they shouldn't use it.
Thank you for your idea. One thing to note is that it is currently possible to remove a custom field from a layout, but still report on it. We would recommend leveraging reports in the case that you still need access to the information in custom fields that are no longer being used.
At this time given the option above, we are unlikely to implement this idea. We hope you can understand.