Is it possible to use a custom SSL certificate with a site hosted via Github Pages?
This is unfortunately not possible directly. GitHub Pages only supports Let’s Encrypt certificates and the provisioning is all managed (and fixed).
Out of curiosity, what’s your use case? This has only been asked once in the past (as far as I can tell) and it was for hosting a Samsung TV app.
If you really need your own TLS certificate, you may be able to do that with Cloudflare assuming you can delegate your entire custom domain to them.
Just a request from my organization’s security department, hosting static pages via github pages and they would prefer to use a cert provisioned internally.