This is not really accurate to my experience using GitHub. In pip for example while we have distutils-sig and a pypa-dev mailing list we hardly ever use them for pip focused discussion. The vast bulk of our discussion (including quite long ones, and I think most folks who end up in a discussion with me know I can produce a fair amount of content in one) occur entirely within GitHub and it works just fine. I don’t think this is unique to pip either. Pretty much the only time we use anything but GitHub are when the blast radius for a change is larger then pip itself (e.g. something that touches pip, setuptools, and pypi) which we use distutils-sig for, or when something is just a notice that doesn’t require discussion, which we use pypa-dev for.

I agree that there are benefits to separating code review and issue tracking (although I’m not a purist about it, I think some PRs are too small to warrant an issue for instance) and I think that without some effort to automate and write a bot GitHub issues are not a good fit for replacing bpo. However I think it’s going to be a regular struggle to get people to try and primarily use bpo for issue discussion (vs code review) because the friction of doing so is fairly high. I think if you want to encourage people to utilize bpo better, your best bet is to do everything you can to reduce that friction.