I wouldn’t say unnecessary; most available comment systems are either a Discuss-style firehose of tracking scripts, or a pain to maintain when self-hosted.
Then how come Github shows you the photocopying page, sometimes for quite a while? It takes long enough to convince me it does more than set a pointer.
> Very early on we figured out that actually forking people’s repositories was
not sustainable. For instance, there are almost 11,000 forks of Rails
hosted on GitHub: if each one of them were its own copy of the repository, that
would imply an incredible amount of redundant disk space, requiring several
times more fileservers than the ones we have in our infrastructure.
I don't know, but since GitHub allows you to show commits from any fork with a URL referencing another fork, GitHub seems to use a common object store for all forks.
Maybe setting up the separate issue tracker and Pull Requests takes some time?