This allows a developer to add a @cached_property to a method with the @abstractmethod decorator, without breaking the check for abstract methods on ABC instantiation. That is, if you tried to instantiate an ABC with a method that had a method decorated with @cached_property and @abstractmethod now, it would succeed, instead of throwing a TypeError as you might expect.

As for why you'd put @cached_property on an abstract method, it's useful for IDEs and type checkers to know that the method is implemented with a property, and that users of the method (and its implementations) can reasonably call it multiple times and know caching is occurring.