I don’t actually understand what that means.
WHATWG specifications are developed through direct open community participation in the form of github issue-tracker discussions and pull requests.
So I genuinely don’t understand what discussions aren’t already enabled by the existing infrastructure of the actual existing spec, or what kind of discussions would be enabled by creating a forked copy of it. I mean, the WICG also just uses github repos and issue-trackers and pull requests, right? (As many or most W3C working groups now do.)
I realize there’s also this Discourse instance as an additional discussion forum. But you obviously don’t need to fork a copy of a somebody else’s spec just to have a discussion about it here.