Is your feature request related to a problem? Please describe.
Currently in order to use GitHub as a backend, you need to run a separate server to facilitate the authentication: https://www.netlifycms.org/docs/github-backend/

Running a separate server is not required for GitLab because it supports PKCE: https://www.netlifycms.org/docs/gitlab-backend/

It would be great if we could do the same for GitHub, so I'm trying to gain attention and drum up support for GitHub to implement this.

Describe the solution you'd like
I've opened a discussion on GitHub, but it would help to get more community support. Please consider up-voting, liking, and commenting on the thread if this is something you'd like to see as well: https://github.com/orgs/community/discussions/15752

Describe alternatives you've considered
The alternative is to run your own external oauth client, but one of the main advantages of running a Jamstack site with a git-backed CMS is not having to run a server, so IMO this defeats the purpose.

Additional context
Other projects that this could be relevant for are TinaCMS and my project Plenti