#if Constantine exists in DW he'll be a passing mention that serves to make the world feel larger and lived-in

[threewaysdivided]

So what we know so far is that the justice league as a whole never new about and ghost or the infinite realms, but did Constantine and other magic users now? Does Dr. Fate?

Those are some interesting questions!

And now, I'm afraid that in order to properly answer you I'm going to have to be a bit annoying.

Truth is, I'm one of those fanwriters who just... doesn't have consistent universal headcanons for a lot of things. There are some common headcanons I use to answer recurring questions (e.g. finding an explanation that accounts for both DP ghosts and DC ghosts in a crossover universe) but in a lot of cases the lore/theories I have about how the world of a specific story works will depend on how I need things to work to suit that specific story.

As for the explanations in Deathly Weapons:

When it comes to the League as a whole, you're completely correct. The standard headcanon I use is that portals to the Ghost Zone/Infinite Realms are very rare and unpredictable outside of a few hot-spot locations (Amity Park, the Bermuda Triangle etc.), which means that actual ghost sightings throughout history have been incredibly uncommon and easily confused with other paranormal/ magical occurrences or mixed in with urban myths/ folklore/ cryptid stories. Add to that a few... interested parties with a vested interest in keeping it that way and League was left with so few data points that no-one even realised there was a pattern until Phantom called Batman and Robin's attention to it.

With regards to Doctor Fate, my take (at least in the DW!Verse) is that the Lords of Order and Chaos are at least aware of the Ghost Zone as another dimensional plane. But since the residents aren't really involved in their conflict and the Zone itself is too big and varied (some areas ordered, some chaotic) to be useful in that conflict or worth the potential costs of aggravating them, they mostly just leave each other alone. I would expect that the average ghost would be more wary/hostile to the Lords than the other way around, since I usually go by the rule that magic is something that can affect them - which for a regular ghost means potentially enduring a very one-sided stomping before being forced into whatever humiliating/ condescending/ cruel situation the Lord feels like.

I also have the idea that Clockwork and Doctor Fate personally know/know of each other and have a kind of passive-aggressive grudging-respect working relationship on the rare occasions their paths cross (both seeing the other as a melodramatic interfering busybody), which isn't relevant to any story idea but I think is funny.

Now, as for Constantine and other magic users...

Let me answer your question with another question: Does John Constantine need to exist within the world of Deathly Weapons?

I swear this isn't meant to be condescending. It's just that I don't really see the point in explicitly including characters if they aren't actually going to interact with the plot/ lore/ character arcs of a story. I legitimately don't know whether Constantine exists in the DW-version of Earth 16 but it's kind of immaterial because as far as the plot of DW is concerned he never comes up. If you'd like to imagine that he does and he's too busy dealing with some unrelated side story of his own to show up then feel free. And if you don't want him there at all then feel free to imagine that too.

This is probably going to sound heretical to some comic books fans but I'm okay with each story being it's own separate thing, and I genuinely don't think it's necessary to for every DC character to exist in every story. In the 2004 Teen Titans Animated Series the Justice League never shows up and is barely mentioned outside of some indirect references. Most of the League (and Earthly heroes in general) are barely present in Green Lantern: The Animated Series. I'm fine with Batman: The Animated Series just dropping Jason and making Tim the second Robin because the writers couldn't/didn't want to/ didn't feel like including a version of Death in the Family as part of that story.

I'm fine with a story or series or show just exploring one character or team in one city and leaving it completely ambiguous whether the rest of the DCU is present in that world. It's also fine by me if a story is taking place in a universe where certain characters exist but never had a reason to become the heroes/villains they are in main continuity. I can accept a world where Harvey Dent became a (perhaps slightly intense) lawyer and nothing went wrong. I can accept a world where a character named Barbara Gordon exists but decided to become a librarian or a gymnast or a private investigator or what-have-you. I can accept a world where Barry Allen or Wally West is the first and only Flash. I can accept a world where Harleen Quinzel declines to apply to Arkham and instead takes work at a small clinic that the Batfamily interact with in passing. To me those are just fun Easter Eggs and if that's all the relevance they have to the story then that's perfectly fine.

I know I'm slowly becoming the sole occupant of the tiny Tumblr niche that is "complaining about scope management" but that's kind of where my brain goes; if a story element or character isn't contributing to the narrative relative to the amount of time/space they consume, then what's the point of having them there?  References and Easter Eggs are a lot of fun, but a collection of references and Easter Eggs does not a story make.

(It’s that “narrative efficiency” thing I keep banging on about.  If you explicitly add a character then you have to add in-universe reasons for why they aren’t participating in the story.  And then more reasons to justify why they don’t just solve the big plot conflicts that are more narratively interesting to have other cast members tackle, but which they would obviously be a better/ safer/ more effective solution to within the rules of their fictional world.  Adding the character means taking time away from the actual story to fix a problem that never needed to be caused in the first place.) 

There are other story concepts I've kicked around in my head where Constantine does exist and does interact with DP and other DC characters but even then, what his role is and how much he knows changes depending on what he needs to know to make that story work.

So I guess, TL:DR - in Deathly Weapons DP-style ghosts are mostly a mystery to the relevant characters, and for others who might exist "off-screen" it could simply be a field of science/magic/study that they aren't personally interested in pursuing.

Sorry for the unsatisfying and weaselly answer, but that's kind of the best I can give you on this one.