You can find the draft at https://www.mediawiki.org/wiki/Wikimedia_Engineering_Architecture_Principles.
You can find past discussion on the talk page, but please put any new comments on the phabricator ticket where we track the last call, as required by the RFC process: https://phabricator.wikimedia.org/T194911.
Great, I linked to the wrong ticket. Here is the correct one: https://phabricator.wikimedia.org/T220657.
At least I linked the correct draft ;)