I don't think it has been strictly enforced in the past, but I also think we should start requiring it. I know I completely forgot to update the release notes for Trees, which I should have done... Perhaps we should provide a very simple module checklist of what we require for a module to enter the release process, really just as a guideline to help developers remember everything that needs to be done.
Some of us had started adopting the convention of placing notes in a file named RELEASE_NOTES.txt in the root directory of each module.