Can I give a suggestion to change the method name to unregister instead so the naming convention is kept in the same family so to say. Would also suggest the code to be more forgiven for odd behaviors (unlikely with this code but nonetheless a great failsafe).

Code suggestion:

    def unregister(self, name):
        err = None
        if name not in self._registry:
            err = f"Client {name} not found in registry."
        else:
            del self._registry[name]            

        if name not in self._clients:
            err = f"Client {name} not found in clients."
        else:
            del self._clients[name]

        if err:
            raise KeyError(err)

The method create_client first checks if self._clients contains name and if the unregister method do not finish deleting everything, you could end up with a half-deleted state where create_client returns a client that the registry does not have.