Hi there!
We have this thing called the Review Queue: https://www.mediawiki.org/wiki/Review_queue
Basically for things that were written by non-WMF feature teams (either volunteers or WMF staff 'other time' stuff). We need to review these things for their performance impact, security, and of course design/ux considerations.
That page links to a general document about the reviews here: https://www.mediawiki.org/wiki/Writing_an_extension_for_deployment
The section on Design: https://www.mediawiki.org/wiki/Writing_an_extension_for_deployment#Design_re...
Which links to: https://www.mediawiki.org/wiki/WMF_Project_Design_Review_Process
Is that page geared for consumption by third-party developers? Or just internal WMF documentation? Is it clear about what things you want from an eg extension author who wants their extension to be deployed on Wikimedia project sites?
Greg