Seems like new version of graphql-subscription uses asyncIterableIterator, so correct usage is:

 return pubSub.asyncIterableIterator('chatSent');

This is noticeable by inspecting type definitions:

PubSub extends PubSubEngine

PubSubEngine class implements:

asyncIterableIterator<T>(triggers: string | readonly string[]): PubSubAsyncIterableIterator<T>;

In npm package "graphql-subscriptions", both asyncIterator and asyncIterableIterator are used (description is probably not updated).