template_preprocess_page has a soft dependency on node module

A quick fix could be to do the proposal to move the variable from template_preprocess_page to node_preprocess_page. I'm not sure if we should remove this altogether since based on anecdotal evidence, this may be used by many custom themes.

If we added an entity-agnostic alternative, we could deprecate the node variable (obviously need to have deprecation support for twig variables first).