As T9148's (or possibly T94414) solution went live, there is an undesirable side affect; no way to disable this new enhancement.
Problem is that now if you watch a category, say for purpose of watching if the category description is changed, your watchlist may be flooded with category additions and removals that you don't care about. Please add an option to the HIDE: menu (prefer default ON) for "category membership changes"
This should probably have an option on both the watchlist itself and the "Watchlist" section of preferences, similar to the minor edits/bot edits/my edits/anonymous edits/logged in edits/Wikidata edits options.
There should also be a checkbox in the "Hide" section of the Watchlist page to hide/show removals and to hide/show additions to categories. Often I only want to see one or the other.
This feature already exists, so I'm closing this as "Invalid".
A deployment error caused this category watchlist feature to be accidentally enabled for a short time on some wikis. This meant that category add/remove events from the time the feature was on are visible on people's watchlists, but because the feature is now disabled, the UI to show and hide them is not displayed.
No add/remove events after about 00:01 UTC will be displayed.