I haven't read this thread (sorry), but the problem with these ideas is always in the execution. Getting all the right people looking out for the changes, tagging them, and documenting them.
Right now we have process for tagging breaking commits, tagging regressed issues, tagging release notes, and these processes are followed by those who know, when they can, but not with perfect consistency. Just doing what we already are is a great burden!
Augmenting that with a human going in, understanding all of them, and writing coherent documentation, is a huge task.
Edit: I also try to point out the most relevant breakage in the release notes.