Secondary browser contexts seem to not be implemented by browsers other than Firefox with rel="sidebar" links.

window.sidebar is planned to be removed which appears to be the only public API to such a secondary context.

From the tests that I have done it seems that all browsers besides Firefox treat links with rel="sidebar" as a normal link. This appears not to have changed in situation since 2009: https://blog.whatwg.org/the-road-to-html-5-link-relations#rel-sidebar

So if there are no implementers interested and it seems to make sense to remove it.