I think Chrome's reasoning for this is completely fair:

> navigator.clipboard is only intended to be used in a focused document, which is not possible for service workers. Therefore, I don't anticipate any intended support for navigator.clipboard.read on service workers

I don't think a browser extension, which I personally consider it sit in between "native app" and "website" in terms of privilege hierarchy, should be able to arbitrarily read the operating system clipboard silently in the background.