It was not practical to use hg and git simultaneously. In my view, and some others, apparently, github was not fully ready to use when the switch was made. I would say not until Miss Islington started doing backports. (They still take more time than on hg, but the difference is tolerable.) The result was that many people stopped patching python for various periods.

It would be practical to have both issue trackers in use for a short time. So I think disabling bpo should wait until the new system has been used and approved by people other than the creators. Unlike with the repository, testing can be done with real issues for cpython PRs.

So I think approval of this PEP should be provisional, subject to final acceptance review, which would then signal 'stop using bpo'. At that point, shut off new registrations and issues, (which could be discourage earlier), and new comments. 'Readonly' should mean that search still works, and that issues can still be closed instead of transferred.