>> Now pushed to master in the commit 3573ebfa6d (it seems this is
>> backward-compatible since it only sets buffer-local variables).
>
> Nice.
>
> Do you plan on adding an outline-[minor-]mode command to hide/show by
> regexp?

So now here are these two commands:

  / s   outline-show-by-heading-regexp
  / h   outline-hide-by-heading-regexp

Also there is an additional helper function that is needed
to keep hidden outlines and restore them after reverting the
xref buffer with 'g' (xref-revert-buffer).
This is an example of advice that does this.
Later when xref will use revert-buffer-function,
this advice could be replaced by a simple hook call:

#+begin_src emacs-lisp
(define-advice xref-revert-buffer (:around (ofun &rest args) keep-outlines)
  "Keep hidden outlines after xref revert."
  (let ((regexp (outline-hidden-headings-regexp))
        (value (apply ofun args)))
    (outline-hide-by-heading-regexp regexp)
    value))
#+end_src