A new Outliner plugin for Obsidian is available here: https://github.com/vslinko/obsidian-outliner

For reference, previous messages mentioning this:

https://www.outlinersoftware.com/topics/viewt/9384
https://www.outlinersoftware.com/messages/viewm/39478

It should be noted that the developer of the Obsidian outliner plugin has separated out the zooming/hoisting/focusing functionality into a separate plugin:

https://github.com/vslinko/obsidian-zoom

(Actually, from my perspective, the hoist functionality is the key functionality, because Obsidian already has folding built-in.)

The only thing missing is to have the plugin work on Obsidian mobile...