This is required to implement add-ons such as Link Status Redux. Currently legacy add-ons replace setOverLink to achieve this.
This has been added to the March 7 WebExtensions Triage mtg. agenda. Agenda: https://docs.google.com/document/d/1zzfedbTKAHUmm4UctxSOIJV3iKayXQ8TuXPodXW8wC0/edit#
We discussed this, and came to the conclusion that it's not worth the complexity it would entail, particularly as the status line has become less prominent in recent years, and related features like `window.status` have been disabled. If you need this sort of functionality, please consider showing the information with an in-content UI instead.
To get in some extra weight: Status-4-Evar is an add-on with ~140k users, which probably also needs an API like this. The use case for S4E is: hiding the default status pop-up, and moving it into a bottom toolbar. Having UI elements pop over the page is often very annoying.  https://addons.mozilla.org/en-US/firefox/addon/status-4-evar/