I am now even more convinced than ever that the "horror theme" was a deliberate red herring.
Consider hexblade. This was originally an "Elric" themed subclass, and was in Xanathar's. Calling it "horror" is tenuous. If it had appeared on it's own, without "Horror" in the title box, no one would have inferred a horror book was in the pipeline.
But there is more than that. There is an artificer subclass. The artificer base class is in the Eberron book. You are not going to put in a hypothetical Ravenloft book: "You must own the Eberron book to use this subclass". Ergo, whatever book the Reanimator is in must reprint the artificer base class. That certainly seems to point to a Tasha's style core rules supplement. The Reanimator being the only way to make a balanced pet based necromancer subclass in 5e. So it fills a demand.
Missing wizard subclass: clearly fits a Tasha's style book.
Then there are several unrepresented base classes. I would infer that is because, whilst those classes will get new/revised subclasses in the upcoming book they could not be described as "Horror themed".
It's also possible that a couple of subclasses have been included from other sources in order to further muddy the waters. For example the folk horror ranger could fit in Shadowmoor.
TDLR: Most of the "Horror" subclasses are for a Tasha's style general core rulebook, scheduled for early 2026. The postponed book that was originally scheduled for October was the MTG tie in.
