I think it should be fixed. Topics about Helperviews, e.g. Expressions, and Changes in the Layout Policy, e.g. extending CSS-Classes for body-tag, are available in the Docs.
Hi folks - let's not ask Philip to try to fix this on his own... he's done a superhuman job already, so all you brainiacs out there: please jump in! At least give us a sign you're taking a stab at it so it doesn't feel quite so lonely.
@1letter I think this is not related to helper views or attributes on the body class.
But why I'm responding, the Plone documentation section you refer to is outdated. Also for other who read this: since Plone 5.1 you can use an IBodyClassAdapter from plone.app.layout to register your own extensions to the body class attribute without having to override the complete LayoutPolicy. As an example, see this part in plone.app.mosaic, but it is using the first implementation: https://github.com/plone/plone.app.mosaic/blob/master/src/plone/app/mosaic/layoutpolicy.py