The guideline says you must add to release notes when you fix a bug, but in practice this is almost never respected, especially because with git it makes merge conflicts almost certain. Automatic updates of release notes or other variants (like herds of slaves doing it in the devs' stead) are currently just dreams.
To the few who care and watch this page: remember that release notes become even more important if you remove the bug numbers from the first line of commit messages.
See also Thread "Git release notes merge conflicts" on wikitech-l (April 2012). Several ideas for a format and automated tool have been proposed there.
However the discomfort and some people neglecting to include release notes does not justify anything. Until and unless we have a different tool, one is expected to include release notes. You'll just have to deal with it, whether you like it or not. If you don't then the change doesn't get merged (it is trivial to amend a patch and add release notes).
You claim of it being "almost never respected" is simply untrue, there are only a small number of changes from relatively few people that don't provide release notes when they should. Most people do when they should.
By the way, I don't see how Talk:Gerrit/Commit message guidelines#Bug number in commit messages is relevant? If anything, moving them from the subject to the body keep the raw git-log easier to scan. More on that over on that talk page.
I agree that release notes are important, but your "small number of changes" is just wishful thinking.
As of now there are over 350 bugs marked fixed since 1.20 was branched, and only 80 mentioned in RELEASE-NOTES-1.21 (which seems to have about 150 bullets in total)... of course it's just approximations but it seems clear that only a minority of bug fixes follow the convention.
I mentioned this topic on the thread Who is responsible for communicating changes in MediaWiki to WMF sites?, I think it bears some relevance on that matter too (but it's a separate one).
Historically we did not (and presumably still don't) include release notes for a bug fix that was introduced in the same release as it was fixed. Some of these bugs may be that sort of situation.
This should be noted in the conventions, anyway it would leave us with at least 190 bugs fixed as of now (those filed before the branching) plus 27 enhancements (requested after the branching), and 188 reports that may need or not be mentioned.