An optional PostMessageOptions object is being added to the postMessage() function for 6 of the 7 interfaces where it’s supported, specifically, DedicatedWorkerGlobalScope, Worker, ServiceWorker, ServiceWorker, and Window. This gives the function a similar interface on its definitions and allows it to be extended in the future. Since broadcastChannel.postMessage() doesn't take additional arguments (such as transfer) it is not being changed.
Docs: https://github.com/w3c/ServiceWorker/pull/1344
Samples: https://gist.github.com/dtapuska/bd92755849991c2fd9b87e1d46691321