> Github really needs to allow people to disable the "Pull Request" tab on repos, to reduce the stigma around not accepting pull requests.

It's more of a workaround than a solution, and depends on how much you trust GitHub Actions, but this one will autoclose all issues and/or PRs.

https://github.com/marketplace/actions/repo-lockdown

> Best solution is to keep code somewhere other than github. Is sourceforge still around?

Aside from Sourceforge and self-hosting (simply via gitweb, or with GitHub-like features in Gitea, Phabricator), there are the big ones like GitLab and Bitbucket, cloud repo services on AWS and Azure, and smaller/indie services like Sourcehut (sr.ht), Launchpad, and RhodeCode.