API (joplin.workspace) states:
"The workspace service provides access to [...] the currently selected notes or notebooks", but there is no method to get the selected notebook, as far as can be seen.
Such feature is important to get the exact environment of the user at the time the plugin is invoked or triggered in any way.
For now you could get it by checking the parent_id key of the selected note, but it's true that a selectedFolder method should also be present. I'll add one.