I'm not interested in hypotheticals. In AAA games that you have worked on, concretely what 3rd party code did your servers rely on that would prevent you from distributing either the server itself or sufficient description of the servers' behavior to allow a reimplementation?

And even if we're talking hypotheticals: stupid example. I haven't worked on a backend where the actual server infrastructure wasn't open source, trivial to open source because it was first party, or irrelevant because the only thing that matters would be the API and protocols, which again, trivial to make open.