extra_navbar doesn't work

[zmoon]

Describe the bug

context

It is in the reference, but in the current state of the repo that is the only place extra_navbar appears in a search.
Maybe it did work in the past, but I don't see anything in the changelog about removing it.

bug

sphinx-build ...
...
preparing documents... WARNING: unsupported theme option 'extra_navbar' given

Reproduce the bug

html_theme_options = {
    "extra_navbar": "",
}

List your environment

Book theme 1.1.0, Pydata theme 0.15.2