What's the relationship with Architecture guidelines? Should that be archived now? Or interlinked with this document?
Topic on Talk:Wikimedia Engineering Architecture Principles
You are right that the Architecture guidelines should be mentioned and the relationship clarified. The guidelines are more concrete, and mostly about how to make changes to MediaWiki core. They are less about what we are trying to achieve. So I think they are much closer to what I have been writing on User:DKinzler (WMF)/Software Design Practices than to the principles. I'll look into consolidating these (as well as Manual:Coding_conventions/PHP).
I have added a link from the guidelines here. I'll link back to them from the non-normative section of the principles.