Over the past few months the TCB team at WMDE has been working on re-factoring code in core surrounding watchlists.
You can find a full blog post about what we did, why we did it and how we did it at the link below: http://addshore.com/2016/05/refactoring-around-watcheditem-in-mediawiki
tl;dr This was work put into making introducing expiring watchlist entries easier. Code was removed from various API modules, special pages and other random places. The new extracted code now has basically 100% unit & integration test coverage.