That feature will not be ready (or at least, not tested enough) to be officially part of 2.80 beta. So we disable it by default, hidding it behind a startup option (`--enable-static-override`), and a python app var (`bpy.app.use_static_override`). That way, people who really want to play with it can do it easily, while not exposing/enabling non-production-ready feature by default. Note that underlying override code remains active, i.e. files we do have overridden data-blocks will be loaded correctly according to static override.