Adding content to Page settings rather than Block settings

Ahh — I found it. It is called the PluginDocumentSettingPanel and can be imported from @wordpress/edit-post.

import { registerPlugin } from '@wordpress/plugins';
import { PluginDocumentSettingPanel } from '@wordpress/edit-post';
 
const PluginDocumentSettingPanelDemo = () => (
    <PluginDocumentSettingPanel
        name="custom-panel"
        title="Custom Panel"
        className="custom-panel"
    >
        Custom Panel Contents
    </PluginDocumentSettingPanel>
);
 
registerPlugin( 'plugin-document-setting-panel-demo', {
    render: PluginDocumentSettingPanelDemo,
    icon: 'palmtree',
} );