We don’t plan on adding additional outlets to the sidebar at the moment. We’re likely going to remove the footer outlet at some point as well.
The new sidebar nav was intentionally designed to be customized via the API so in the event that it changes in the future, we can more easily migrate to a new structure. The idea being that the navigation area isn’t something we want any random content injected into.
This isn’t to say the sidebar shouldn’t be customizable, but we want to expose more specific APIs rather than rely on open-ended plugin outlets.