RelatedTitle Flarum's homepage uses GitHub pages, which sadly doesn't offer an easy way of using HTTPS on custom domains... I know they are using a CDN to serve all GitHub Pages websites, and it would probably cost them too much to upload custom certs for every domain there. I hope they will offer a solution to this as it's getting the norm to serve on HTTPS.
I believe GitLab has Let'sEncrypt integrated with their static hosting, but they aren't using a CDN like GitHub.
Flarum could use Cloudflare to have a certificate for the main domain (that's what I use for all my GitHub Pages websites) but I guess they decided not to use it. Using Cloudflare for that does not completely solves the issue as Cloudflare won't validate the certificate between Cloudflare and GitHub.
Safest method would be to setup an Apache/Nginx proxy (with Let'sEncrypt or Cloudflare HTTPS for example) which answers for the main domain and validates GitHub shared certificate to the origin.