Some properties that want to allow either relation, from Template talk:Constraint:Type:
- P1080 “from fictional universe”: Most things are instances of some “fictitious entity” subclass, but “lightsaber” is a subclass of “fictional sword”, not an instance of anything.
- P1422 “Sandrart.net person ID”: some “persons” are groups that, in Wikidata, are represented by a class item.
- P306 “operating system” (value type constraint): an operating system, like Linux or Windows (yes, I know, OS ≠ kernel, shut up), or the special value “cross-platform”. “cross-platform” isn’t an instance of anything, but it is at least an “indirect” subclass of itself (path length 0), so the relation “instance of or subclass of” would permit that value, which the current relation “instance of” doesn’t.
- P705 “Ensembl Protein ID” needs this, because its items are either proteins (instances) or families of proteins (subclasses). There are a lot of items using this property.
There’s already an item for this relation (Q30208840), and apparently some constraints are already using it. But I don’t know if this feature is high priority.