It seems to me, public paper discussion of proposals is far from perfect.
People have few options:
 - Mail author. Not public, so not really an option
 - Reddit. Bad discoverability. No guaranties, the author will notice the comment(s). Comments are locked after some time.
 - This list? In any case, not great either, because it is A) For future (unsubmitted) proposals B) Not really for regular cpp users, but for experts. In a way, the list itself is also not really discoverable.

Interestingly, the papers themselves are fairly easy to discover (and track) via both cplusplus/papers on github and of course the open-std page.
However, once discovered there is no way to comment on them in a way, the comment will be visible to both the author and the general public. 

Having a central place to comment on paper will also solve another issue we (the users) have - tracking design progress after a committee meeting. The only source are the trip reports, which are also hard to discover, if available at all. 

Lastly, having a place for public discussion does not replace current forms of communications, nor should it be "mandatory" (the author can decide not to participate and should be able to remove comments). However if the author is OK with a public discussion, there should be a place where the users can find this discussion and participate in it. (One can argue, if the paper is on the author's github, there a comment can be made, but this will be unstable in the long run as this will be a comment to the file, not the paper - it might change)

Thank You