Yes, They Are On-Topic
Full Disclaimer: I am the user who posted the Spider-Man question
I would argue that questions regarding legal reaction to supernatural phenomena are on-topic for Worldbuilding SE, because they involve phenomena and legal situations that are out of context for modern law. I make this argument for several reasons:
1) We already have a law tag
There is already a law tag on SE, whose description is "For questions related to the sets of rules set in place by organizations to govern the behavior of the people". This by itself would suggest that a law-related worldbuilding question is on topic for Worldbuilding SE. Additionally, we have specific tag for how people enforce said laws (law-enforcement), which is dependent on whether individuals broke laws to begin with. The tags for society, social-norms, secret-society, and government also frequently involve legal aspects of how society works. There are also many, many questions on Worldbuilding SE about how the law would interact with supernatural phenomena:
Taxing the superhero industry
Why do governments not execute or otherwise permanently contain super villains?
What laws, if any, would have to change to allow people with superpowers to use their powers for profit in companies/businesses??
Humans can now fly. What new laws are created? (many of the comments and answers involve discussions of existing law)
Where could a vampire get blood without harming people?? (Am also the author of this question. Involves a lot of questions as to the legality of various actions)
How can a banker successfully (and safely) lend money to an autocrat?
Dragons and aviation bureaucracy
Additionally, we have a large number of questions related to how existing geopolitical structures would react to supernatural (or otherwise "fictional") phenomena:
What is the ideal location to hide the President in the case of a zombie apocalypse?
What is the impact on the world politics if teleportation is possible?
Geopolitical and Economic Implications of a Large Eruption at Yellowstone
And so on.
2) Law is a fundamental part of worldbuilding
Worldbuilding, at its core, is a question of how the real world interacts with the fictional one. This is what differentiates Worldbuilding SE from Writing SE, Writing SE is about how fiction interacts with itself; Worldbuilding SE is about how fictional elements would interact with reality. Law is a part of reality, it is the rules on which how society interacts, as important a factor to consider as geopolitics, sociology, psychology, physics, biology, or chemistry. We have many, many questions about how supernatural elements would interact with the laws of physics, the laws of chemistry, the laws of evolution, the laws of human psychology. Why are the laws of humankind exempt?
Indeed, law and society are fundamentally the same, all law represents is a codified set of societal rules written down to determine what behavior is and is not acceptable by a group of people, modified by historical precedent.
Legal reaction to supernatural phenomena is part of how society reacts to that phenomenon. We have tons of questions regarding "how would society react if superpowers were real" or "how would people react if vampire are real" or "how would people react if aliens were real". Law is a part of that. Frankly, the only difference between geopolitics and law in a broad sense is that geopolitics involves the rules and interactions between nations, whereas law concerns interactions of civilians within nations.
Both @elemtilas and @KeizerHarm consider questions that involve entirely new laws written in regards to some fictional phenomenon to be on-topic. The thing about law is that it is usually reactive, laws are only passed in hindsight once someone discovers a loophole in the existing legal structure, or more rarely if someone has enough forethought to notice an event isn't covered by existing law. Until someone passes anti-vampire or anti-superhero legislation, societal structures are going to react to either based on pre-existing legislation (or break existing laws, which in and of itself involves legal questions).
Related to this, the secret-society tag and other tags involving superhero and urban fantasy settings are going to heavily interact with the law as it is written IRL, because a key feature of both of these settings is they are set in a setting which is "reality unless noted", and the presence of supernatural phenomenon has a minimal effect on legal structure because the supernatural is hidden or poorly known. Therefore any legal action on behalf of society against supernatural phenomena will have to be based on existing legal precedents, most countries today have laws against ex-post facto laws, so it would not be possible to pass a new law banning a supernatural activity and then retroactively apply it to the setting.
3) Posting worldbuilding questions as “real world analogies” in Law SE strips them of their context and results in lower-quality answers
@Elemtilas suggests asking law-related question on Law SE by rephrasing them as "real-world questions with the supernatural elements stripped from them. However, doing so means stripping the questions of valuable context that would influence how an IRL legal professional would address the issue and thus reduce the quality of the answer. A general trend I have noticed on most stacks (including Worldbuilding SE), is that the more detail you include in a question, the better. If you don't, you often end up with an answer to a question that is technically correct but does not address your problem.
This issue can be demonstrated with two of the questions being brought up: the Spider-Man question and the one regarding if summoning an eldritch being violates zoning regulations. For the first, it’s true someone could buy a wingsuit and go parkouring through New York. However, Spider-Man also leaves significant amounts of webbing wherever he goes and is capable of swinging through New York City at speeds of over 100 km/hr. Someone with a wingsuit does neither of these things. A spider-themed superhero might face littering charges for their webbing, which a wingsuit-wearing person would not.
Many superpower questions in general struggle to be applied to "real world analogies" because there isn't really an analogue for them in modern terms. There are a large number of questions on here about settings where people are born with superpowers of some kind. They can't really be converted to real-world questions as "people with military-grade weapons" because people aren't born with AK-47s in their hands, nor are these weapons a fundamental part of their body that cannot be removed without invasive surgery.
As another example, take the “would summoning an eldritch abomination result in zoning violations question”. It can't really be converted to an IRL example of a 12-car garage because there are fundamental differences between the two questions. Notably that an eldritch abomination is alive and is often capable of movement, it isn't a fixed structure in the ground. It's not possible to ask an IRL analogy question on Law SE because in real life there is no mobile terrestrial organism the size of a building.
Other common questions I can see that cannot be easily asked on Law SE are "do inalienable human rights apply to sentient aliens", "is killing a vampire murder because they are already undead", and questions regarding telepathy like "is telepathy you cannot turn off a violation of privacy laws" or "is evidence garnered through telepathy valid in court". Even in cases like telepathy changing it to something like a precog machine misses context, since telepathy would be an innate quality rather than a device.
There are many cases of law in fiction that can be asked on Law SE with little problem. For example, many "realistic fiction" settings like Tom Clancy novels there are no supernatural elements, everything is theoretically possible IRL. But in cases with supernatural or otherwise non-realistic worldbuilding elements, like aliens, superpowers, futuristic technologies which we have no IRL analogue for, these questions cannot be simplified in IRL terms.
4) Other stacks, in general, do not like having questions on explicitly fictional scenarios asked on them.
I'm sure everyone here is very familiar with this. Questions about fictional settings or elements that do not occur IRL are not popular on other stacks. This is the reason why we do not ask questions about creature-design on Biology SE, or questions about superluminal space travel on Physics SE. I know Worldbuilding SE doesn't like the rest of SE considering Worldbuilding SE their dumping ground for every question they don't want, but for once the topic is actually within our purview.
Looking at the question on Law SE, only one person has replied so far and they have done so somewhat positively, but they have already indicated that at least one of the questions OP listed regarding supernatural phenomena and law would not be acceptable there. This does not bode well with shifting all supernatural-related law-based questions to Law SE.
Additionally, one thing I have noticed is that people outside of Worldbuilding SE are less likely to examine the question seriously and provide useful answers. They are more likely to either give a cheeky, non-committal answer like "Where does a 600 lb gorilla sleep? Anywhere he wants" or say "well in fiction you can make anything you want happen".
tl:dr; Legal questions involving explicitly supernatural phenomena and how the legal system would react that cannot be answered in reference to IRL phenomena are on topic, questions of legality of an action for the sake of a story by itself (that is, the issue could potentially come up in real life) are not.