The refactoring started as part of [RFC] Expiring watch list entries. After an initial draft patch was made touching all of the necessary areas it was decided refactoring first would be a good idea as the change initially spanned many files. It is always good to do things properly ® instead of pushing forward in a hacky way increasing technical debt.
The idea of a WatchedItemStore was created that would remove lots of logic from the WatchedItem class as well as other watchlist database related code that was dotted around the code base such as in API modules and special pages.
The main patches can be seen here.