I see the term "pure upstream" used a lot describing different software packages/distributions. I get that "upstream" in the context of open source refers to a code base from which a certain software package was forked. But what does it mean to say that a certain software package is "pure upstream"?
I think the “pure” part of "pure upstream" means that the codebase remains unencumbered with changes that are custom changes needed for a particular application use or environment which isn’t applicable to most users. Some projects may want to evolve to become more of a Swiss army knife of an application but some may decide to keep to a small coherent functional space. How purity rules apply is a judgement call for the maintainers of the upstream project.
An example of a non-pure fork could be a proprietary extension that could cause vendor lock-in. Take a look at this article about Kubernetes: https://technative.io/kubernetes-must-stay-pure-upstream-open-source/