I am trying to configure the GitHub plugin for our discourse-hosted FOSS project site, but the “GitHub client secret” setting rejects the token: github_client_secret: Value doesn't match the required format. My token is in the (not that) new format that starts with ghp_ (see Authentication token format updates are generally available - GitHub Changelog) – could it be that the plugin hasn’t been updated to accept that format ?
I think you are not using the correct setting. GitHub client secret is a core setting to configure GitHub login (to sign in to Discourse with your Github account); see Configure GitHub login for Discourse. It requires the creation of an OAuth App.
What you’re looking for to configure the GitHub plugin is here (type plugin:discourse-github)