It's admittedly not really practical for most projects, but it could be for some large ones - Rust, for example.
But, yes, projects like Rust could have selfhosted if they had wanted to.
However I got the distinct feeling the whole stack would not fit as well into an enterprise environment, nor would the tooling around it work well for devs on Windows machines that just want to get commits out there. It's a perfect fit for that kind of project but I don't think it would be a great GitHub replacement for an enterprise shop that doesn't have software as it's core business
I use a self-hosted GitLab instance in a commercial setting (with developers on Linux and Windows) as well. It's a software department of a non-software company. Fairly small. The person or persons in charge of GitLab have set up some pretty nifty time-savers regarding CI and multi-repo changes - I'd prefer a monorepo, but the integration makes it bearable.
These days sourceware.org hosts the Valgrind git repo, buildbot CI and web site. We could also use their bugzilla. There isn't much point migrating as long as KDE can put up with us.