It is, unless you want to use a custom domain with it. With the latter case your only choice to retain some semblance of HTTPS is to put it behind a CDN, but that still breaks end-to-end HTTPS. The problem really has to solved at Github's end, by allowing users to upload their own certs.
You can set your own cert on the CDN though, and have the CDN request origin objects over https from the github domain. Since the CDN is the endpoint for your domain, you'll still be presenting a custom domain to your users.
Which CDNs support that kind of domain translation?
Wait, how is serving your content through Github's servers using your own certificate any more secure than letting GitHub or CloudFlare provide the cert?
It's arguably more secure than letting cloudflare do it (adding a new party doesn't make the chain more secure) but it doesn't have to be more secure than using githubs cert. It's just that you can't use githubs cert with your own custom domain.