Related to this recent GitHub issue, I tried following the link in a few of those answers to docs.github.com, only to find that Firefox and Chrome have blocked the domain due to a bad SSL certificate: SSL_ERROR_BAD_CERT_DOMAIN. Ironically (scarily), the link in question is the one you're meant to use to verify the (recently changed) SSH keys after running ssh-keygen -R github.com:
How long has the domain had an expired domain for, and is this issue related to the recent GitHub issue?
It appears this has now been resolved, although there's not much more information on it and it appears on GitHub's incident tracker as an incident with GitHub Pages, even though it was evidently affecting more than that:
We are aware of TLS problems related to an expired certificate in our CDN configuration. We are working to resolve this issue.
Either way it seems to have lasted about 20 mins and is presumably unrelated to the wider SSH key leak issue... although two rookie mistakes within the space of 24 hours from a company of GitHub's scale is worrying.