I read the "rules" for W3C groups and in order to actually participate you need to be a member of a big organization and all this other stuff. I'm not sure anyone would have listened.
And if it was sent between the spec becoming a Candidate Recommendation and going to Proposed Recommendation, it must (in theory) have been addressed. If not, something's gone wrong process-wise with how the group was operating (and from poking around a bit, it seems likely it did). Le sigh. :\
From prodding around a bit (notably [0], which sadly is in Member-only space, but plenty of administrivia is there, and in principle no technical work for almost all groups), it seems like every issue reported to the Working Group (regardless of where) should have ended up with a GitHub issue, with [1] being meant to have been all issues while the specs were in CR.
Pointing in the specs to a mailing list to report issues, and then relying on someone to copy them into GitHub, seems doomed to fail: it's far, far too easy for one thing to not get copied. Really the "Status of the Document" should've pointed to GitHub for new issues being filed (possibly with a fallback to the mailing list for those unable to use GitHub for organisational or other reasons).
[0]: https://lists.w3.org/Archives/Member/chairs/2016OctDec/0143.... [1]: https://github.com/w3c/web-annotation/milestone/3?closed=1