See https://www.mediawiki.org/wiki/API:Watchlist.
Feature summary:
Add "wltitle" to watchlist API to obtain the watchlist entry for a specific page.
Use case(s):
Extensions and scripts that show/use the user's current watchlist setting.
Benefits:
Currently a pain and unreliable to obtain this by filtering.
Such a filter would give all watchlist entries for the specific page, but that could be done by list=recentchanges as well.
action=query&list=watchlist gives the full watchlist (Special:WatchList), if you want to know if a page is watched by the current user (and the expiry), there is action=query&prop=info&inprop=watched&titles=, which can be combined with other prop or list or meta requests.
What is the use case to filter the watchlist for a title? And why not just filter the recentchanges by a title?
Hey, that solves it, thanks! The inprop thing, not the recentchanges. Recentchanges is useless for me as the page may or may not have been edited in the past 30 days. May I suggest to change the text "Get basic page information" on https://en.wikipedia.org/w/api.php?action=help&modules=query to "Get basic information and user preferences and permissions for the page"? As the watchlist status isn't a property of the page I didn't expect to find it there. If the description had said this I would have found it I think.
The watchlist status is a property of the page and user requested for. Similiar for permissions or readable flags, which are related for the user. But that maybe a technical view of it. I have no idea if another description helps to find it.
No, things like protection level, talk ID, displaytitle and number of watchers are properties of the page. They are the same for everyone. Watchlist status is a user preference.