In MV2 it was possible to use window.close() in the background script of an event page ("persistent": false in manifest.json) to immediately unload the background page.

It allows freeing up memory when you are absolutely sure that there's no point in keeping the background context alive e.g. if the extension only uses chrome.alarms and doesn't listen to any other unpredictably occurring events. These are simple timer or scheduler extensions that wake up every X minutes to spend just a few milliseconds in the background script to do things like updating the badge/icon/title of the extension action icon or pinging a remote server or other maintenance.

The problem is that service workers don't have a terminate method available for normal dedicated/shared workers because SW is a special kind of worker for a web site, but since it doesn't serve this purpose in an extension this restriction is inappropriate [1]. If an extension wakes up every minute then a half of every minute will be wasted on keeping 20MB of an unused worker.

A possible solution would be to add something like chrome.runtime.terminateBackgroundScript().

P.S. I don't suggest reinstating terminate method inside ServiceWorker object because Chromium team has been against altering the web platform behavior for extensions, exemplified in its decision to add chrome.dom API.

Note [1]: It's inappropriate because extensions don't need specifically an SW to do their job, which is to process chrome API events (completely unrelated to SW specifics) or to show their chrome-extension:// pages (already local). It's one of the many inherent restrictions of the SW technology that's not suited for what extensions do, as we can see from #72 that lists a lot of similarly inappropriate restrictions.