User story: As a patroller, I want to know when a newly created page might have a corresponding draft in the Draft namespace, so that I can check whether a history merge is needed.
If a page is created, and a page in the Draft namespace exists with an identical title, users would like to know so that they can investigate whether the page was copy/pasted into the article, in which case a history merge may be required.
It could, but I think @Samwalton9-WMF's idea is to have this as a tag so that it can be filtered on in RecentChanges, the page creation log, and NewPages and so that people that deal with those get notified that they should probably check the draft (perhaps it requires a histmerge, perhaps content can be incorporated into the article, et cetera).
AbuseFilter can also tag, but I don't know if it's currently possible to write a filter that would do this tagging.
No, it isn't. There is nothing like "check if an arbitrary page exists" in abuse filters.