Crowd Control is one example which offers additional features for both viewers and broadcasters who pay (in the form of subscribing to the Crowd Control Twitch Channel).
From my experience though it’s not something many extensions implement because viewers don’t want to using disposable income to pay a developer they don’t know when it could go to supporting the streamer they’re actually watching. And for the broadcaster, they can already work in partnership with developers by hiring them to make extensions, which is quite common for extensions limited to use on certain channels.