Thanks Mark for opening this discussion, it's very useful.
Indeed, by reading https://www.mediawiki.org/wiki/MediaWiki_1.20 one would think that nobody has been done in 1.20, except perhaps some localisation work. :p I also agree that https://www.mediawiki.org/wiki/MediaWiki_1.18 is a good example to follow.
On RELEASE-NOTES: it *might* be comprehensive enough (quite often things get forgotten), but in my experience it's completely useless to understand what are the really important things, unless one is looking for something in particular (which is, IMHO, the point of release notes besides big deprecation warnings and so on). The /wmfX subpages are perhaps even less useful, as they're just list of commits for now (crucial, but not for all users), although I see that a section "The biggest changes" has been added (mostly empty it seems?). https://www.mediawiki.org/wiki/MediaWiki_1.21/wmf3 Scanning a dozen sub-releases makes one lose the overview of the really important things anyway.
I don't know if free-form tagging in gerrit will solve all our problems, but for now I can't think of anything better than adding a short note to https://www.mediawiki.org/wiki/MediaWiki_1.21 It doesn't need to be more than a [[gerrit:12345]] link, or a link to a topic on gerrit, or whatever. People caring about the page will look for more information later. When you have that pleasing feeling "hurray my new awesome feature/long-waited-for bugfix has been merged", adding a link to a wiki page should be too much of a burden I think?
A simple thing that many software projects have and we seem to be missing is an automatic list of bugs fixed in a release: we only have target milestones. This means that: 1) your only option are release notes, which can be forgotten quite easily; 2) sometimes a bug is fixed in a branch but not in the main release and we have no way to tag it as such.
Finally, we should perhaps also add some information on the main extensions, especially the ones bundled in the release?
Nemo