> So shouldn't outline-minor-mode-use-buttons be just a buffer-local
> variable like all other variables that affect outline-minor-mode?
> Then it could be set in mode hooks like other variables:
>
>   (add-hook 'ruby-mode-hook
>             (lambda ()
>               (setq-local outline-default-state 2
>                           outline-minor-mode-use-buttons t)
>               (outline-minor-mode)))

Here is a patch for this simplification: