GitHub now supports https for custom domains (which was previously only possible with Cloudflare and other solutions):
https://blog.github.com/2018-05-01-github-pages-custom-domains-https/
We're in the process of adding it now. Currently waiting on this step:
Not yet available for your site because the certificate has not finished being issued
Certificate has been issued and https is now enforced!
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Certificate has been issued and https is now enforced!