Archive subtrees under the same hierarchy as original in the archive files.

org-archive-subtree.el

(defadvice org-archive-subtree (around fix-hierarchy activate)
  (let* ((fix-archive-p (and (not current-prefix-arg)
                             (not (use-region-p))))
         (afile (org-extract-archive-file (org-get-local-archive-location)))
         (buffer (or (find-buffer-visiting afile) (find-file-noselect afile))))
    ad-do-it
    (when fix-archive-p
      (with-current-buffer buffer
        (goto-char (point-max))
        (while (org-up-heading-safe))
        (let* ((olpath (org-entry-get (point) "ARCHIVE_OLPATH"))
               (path (and olpath (split-string olpath "/")))
               (level 1)
               tree-text)
          (when olpath
            (org-mark-subtree)
            (setq tree-text (buffer-substring (region-beginning) (region-end)))
            (let (this-command) (org-cut-subtree))
            (goto-char (point-min))
            (save-restriction
              (widen)
              (-each path
                (lambda (heading)
                  (if (re-search-forward
                       (rx-to-string
                        `(: bol (repeat ,level "*") (1+ " ") ,heading)) nil t)
                      (org-narrow-to-subtree)
                    (goto-char (point-max))
                    (unless (looking-at "^")
                      (insert "\n"))
                    (insert (make-string level ?*)
                            " "
                            heading
                            "\n"))
                  (cl-incf level)))
              (widen)
              (org-end-of-subtree t t)
              (org-paste-subtree level tree-text))))))))

I put in the advice in a small emacs package that has a function to enable and disable the advice:
https://gitlab.com/Thaodan/emacs.d-lisp/-/blob/master/org-archive-subtree-hierarchy.el