Debugging issues like T110850 is hard because the user can only determine that he is affected by such an issue usually some time after the fact, and either log files may have been purged or there is the underlying question how reliable a user's claim is that he indeed did not receive a mail several weeks or months ago.

To make that easier, there should be a special page or an API call that lists the pages on the watchlist along with their "notification statuses", i. e. whether when the particular page is changed, the user will be notified. https://www.mediawiki.org/wiki/Manual:Watchlist_table#wl_notificationtimestamp says:

This is the timestamp of the first change to the page made by another user after the last visit by the watching user; the timestamp is cleared when the user visits the page. When the timestamp is updated, notification email may be sent.

The timestamp is always cleared (made NULL) when the user visits the page, whatever their personal settings, as long as either either of $wgUseEnotif or $wgShowUpdatedMarker is true.

One simple idea would be to mark up page titles on https://en.wikipedia.org/wiki/Special:EditWatchlist that have not been visited as bold.

At least for my non-debugging use case, even better would be a special page that lists all unvisited pages with links for the diff since the last visit.