This sounds great but how do we start to improve the situation? Maybe start with filling in Developers/Maintainers so there are no red squares (there are a lot of them now)?
There's also https://phabricator.wikimedia.org/project/view/609/ - should we track it? Should we plan for some kind of regular progress on it, generally or per team?
Per my reply under the Unclear Ownership thread, there's some work that started recently to try and address this. Task https://phabricator.wikimedia.org/T174091is specific to the stewardship work.
A concerted effort may be good, but it may also be overwhelming.
My suggestion would be to at least assign ownership and schedule tech debt wrangling when the need arises. If the editors team runs into an issue with the editpage API, they should take ownership of that API at that point, and find out what it takes to fix it. If it turns out that this needs a change to, say, the revision storage layer, they should talk to the platform team.
Picking one refactoring that we have already agreed on but haven't properly finished (say, extension registration) and actually driving it to the end could be a possible avenue for more organized activity. Right now architectural improvements usually get stuck in "we now have N+1 competing standards" land.