Do you have any references you could give on this subject? I think I have a fair idea of all the object systems you listed (and a couple of others), but I understand very little about ECS. And every time I read about ECS I feel like it should be expressible as some flavour of OOP much as you say here, but I struggle to express that in a concrete manner.

In particular, keeping a single object’s state in several separate memory regions seems atypical (prototypes—or Self’s “traits objects”—are usually stateless even if they don’t have to be); and I can’t think of any way to map iteration over all objects that have parts of a specific kind, often mentioned in expositions of ECS, to a conventional OOP concept (seems relational if anything).