I must say that it does worry me that it is still not scheduled and apparently still unassigned.
My old overlay add-on is complex and will take months to port to a WebExtension. With just 9 month to November where version 57 is released and overlay extensions will no longer be supported, it is a major worry for me that many of the APIs that I need are not yet ready and not even scheduled.
Hope to see this API soon. Otherwise I might not have enough time to get my add-on ready.