> Can this be done out-of-the-box now?

Yes, with built-in packages there are at least two different ways - speedbar, or Dired in a side window. And what even "out-of-the-box" means? In Emacs-land packages are not like extensions in other editors and IDEs, they more like recipe books. You can collect literally over a thousand of them without ever explicitly loading them until that specific moment that happens once in a decade and you won't ever notice their presence until then. And there are tons and tons of different packages that do the sidebar directory listing - treemacs, dired-sidebar, dired-subtree, direx, ztree, etc. You seriously thought that in 50 effing years nobody, just nobody thought of making a sidebar directory tree that works in Emacs?

> Nothing worked seamlessly or reliably.

Skill issue. You. You couldn't figure it out. And then decided it can't be done, period. Brilliant.