@AlexWaygood any thoughts on this?

I think this is a good idea for a rule. I agree that if a class that doesn't have ABCMeta (or a subclass of ABCMeta) as its metaclass, but has @abstractmethod-decorated methods, that's probably a bug. Here's a few things we'll need to keep in mind, however:

1. We need to recognise as valid any class that has metaclass=X, or any class that inherits from a class that has metaclass=X, where X is either abc.ABCMeta or a subclass of abc.ABCMeta.

2. For a class like the example @Skylion007 gives, that doesn't inherit from any classes or have a custom metaclass, it's easy to see that it doesn't have ABCMeta as its metaclass. But we'd need multifile analysis to be able to accurately determine whether this class has ABCMeta as its metaclass, since it depends on whether ForeignSuperclass has ABCMeta as its metaclass or not:

   from other_module import ForeignSuperclass
   
   class Klass(ForeignSuperclass): pass

   What should we do in cases like this? I vote for not emitting the error if a class inherits from any classes that come from external modules, since this is a common pattern, and I think there will be too many false positives otherwise.

3. Stub files need to be excluded from this rule. Legacy code that was written before the introduction of the abc module often has classes that are de-facto abstract base classes, even though they don't make any use of the abc module. When writing type stubs for legacy code like this, you'll often want to add @abstractmethod to the stub methods of these classes, so that type checkers understand them to be abstract methods that need to be overridden in subclasses and emit appropriate errors. However, you won't want to lie about the metaclass in the stub file and pretend that the class has abc.ABCMeta as the metaclass, since this can cause spurious type-checker errors about metaclass conflicts in some situations. Adding @abstractmethod to a method on a class that does not have ABCMeta as the metaclass is fine in a stub file, since type checkers will still recognise the method as being abstract; the metaclass only makes a difference to runtime semantics, and stub files are never executed at runtime.

in fact the @abc.abstractmethod docs says

Using this decorator requires that the class’s metaclass is ABCMeta or is derived from it.