Watchlist can show you all category changes of a watched category. What I really miss is a possibility to only watch additions and removals of an arbitrary category (I don't want to add a category to my watchlist to see recent changes).

Using RelatedChanges/RecentChangesLinked is not useful because it shows only additions because when a page is removed from the category, it is no longer linked, and options don't allow you to only have categorization changes shown.