In the present context, we only really need "impact" for something that is going in to -stable. When it goes into -stable it is a new commit and so the commit message can be adjusted then.
When a patch is being submitted the -stable, the submitter presumably can justify the request for the patch to go into stable. If that justification were written is clear succinct text (the hard part!) and placed in an 'Impact:' line, then these release notes would be already written.