Is this really about "managing" or also about "dealing with and decreasing"? If the scope is not MediaWiki Core only, might case studies provide some insights? (Open question; maybe the answer is a clear "No".) Like killing AFTv5 in the past (still used by 3rd party sites; took a while to properly remove any traces on WMF sites), or unresolved tasks like "Deprecate EducationProgram extension" in Phab:T125618 ?
Topic on Talk:Wikimedia Developer Summit/2017/How to manage our technical debt
I'm not sure I understand your first question. "Managing" is a synonym for "dealing with", and "decreasing" is only one of many strategies. Sometimes increasing technical debt is smart. Great software designers frequently decide that "kludgy but fast and functional" is better than "perfect when it's 'ready'". I also fully agree with @Greg (WMF)'s statement about scope: "Scope is not just MediaWiki Core or even just MW Core+Extensions, but everything (where 'everything' == 'things of concern in the Wikimedia development community')."
1) Scope is not just MediaWiki Core or even just MW Core+Extensions, but everything (where "everything" == "things of concern in the Wikimedia development community"). So, explicitly this includes eg services like OCG.
2) I think case studies/lessons learned would be a great session (either of one or a couple, depending). I'd love someone who is closely familiar with the thing at hand to do it (probably not me, though I'd be more than willing to help prepare or even present if needed).